So, I went through a few hundred posters online, but unfortunately I couldn't find this one, which I was hoping to get a more clear picture, or find the artist who drew this, since I can't make out the two characters at the bottom. If you know the artist, go ahead and reply to this later on, and that might clear some things up. As for now, I'm going to focus on Sino-Mongolian relations during the 1950s, and my theory as to why the painter decided to use different colors.

Sino-Mongolian Relations in the Post-Qing World, 1911-1949

So for a bit of background, Mongolia did declare independence from the Chinese after the Qing fell, but that independence never fully manifested itself until a second revolution in 1921. Back in 1911 there just wasn't enough support among the Mongolian nobility for a full political break with China. Nationalist Mongolians were able to later gain the support of the Soviets after the 1917 revolution, who were willing to send some advisors and materiel they could spare in the middle of their on-going civil war. The gambit paid off, and the Mongolian People's Revolutionary Party officially took over in 1921. But in spite of the Soviet-backed military takeover, its noted that Chinese influence over the country was still substantial compared to Russia's. Estimates of the number of Chinese in the country in a sketchy 1918 census puts them at 100,000 out of 675,000 people, though the real number is probably around 25-30,000 with most concentrated in the capital. In 1925, four hundred Chinese firms operated in Mongolia, comapared to 50 Russian firms. But the 1930s would witness the beginning of what would come to mark Mongolia's foreign policy: pendulum politics.

Finding itself trapped between two major regional (and then worldwide) powers, Mongolians had hoped to maintain their independence by playing one side on the other. In the 1930s, there was a max exodus of Chinese from Mongolia; in 1932 the Chinese made up 10% of the industrial labor forced, down from 63% in 1929. Two things had allowed the Mongolians to wipe out Chinese dominance over their country: The first was the consolidation of Soviet power across Central Asia after the Russian Civil War. The Second was China's ascendancy into chaos. The lack of a strong central government after Yuan Shikai's death meant China was never going to be able to regain control of its peripheral territories. This mostly included all those lands gained by the Qing: Xinjiang, Tibet, Mongolia, Taiwan, and Korea (as a vassal state). Furthermore, Japan's invasion of China in 1937 prevented the KMT from ever being able to organize any real effort to reclaim the territories. In 1945, the three super powers conferred at Yalta, and the Soviets were able to gain support for an officially independent Mongolia by forcing Chiang Kai-shek, who's Nationalist Party still recognized Mongolia as legitimate Chinese territory, to hold a plebiscite in Mongolia. On October 20, 1945, almost 100% of the Mongolian electorate opted for independence (the plebiscite was revoked in 1947 after a border skirmish). Now, Mongolia was both politically and economically liberated from China. It was a new, independent nation!

The issue with this new-found independence was that it came at a heavy cost for the Mongolians. Now, the USSR had 100,000 troops stationed in Mongolia, and exclusive rights to Mongolian mineral mining, the most lucrative industry inside Mongolia. Realistically, while maintaining political independence on the surface, Mongolia had just switched from a Chinese territory, to in the Chinese cultural/economic sphere, to replacing that with Soviet culture and economics. This wasn't really anyway to live an independent life. Matters in the south finally came back up when Mao and the CCP completed the takeover of China in 1949. The KMT may have backed down on the Mongolia issue, but the CCP wasn't so generous, nor were they obligated to. Since the 1930s Mao had been an outspoken critic of Mongolian independence, seeing it as rightfully Chinese. But while the CCP claimed Mongolia as a subject, this also gave the Mongolians an interesting opportunity to manipulate the two powers, the Soviets and China, into taking advantage of each other's claims to Mongolia.

1950-1960: The 10-year Economic and Cultural Agreement, and the Sino-Soviet Split

In 1952 the Mongolian head of state, Tsendenbal, suddenly reached out to the CCP. He wanted to begin warming the once soured relations between the two nationalities and countries. A meeting was set up in Moscow, where Stalin, himself, and Zhou Enlai agreed to the construction of a railroad from the Chinese border to Ulaanbaatar, which then linked up to the USSR. Then, Tsendenbal left for Beijing to sign an agreement on economic and cultural cooperation with China. The deal included a 1955 grant of 160 million rubles to buy Chinese goods and three loans in 1956, 58, and 60 to finance construction projects. This included a textile plant, paper mill, plywood factory, glass works, roads, bridges, irrigation projects, and electric power stations. China also provided thousands of Chinese workers to help carry out the projects, reaching a peak of 12,000 workers in 1956. Despite all this, however, it should be noted that although Mongolia's exports to the USSR fell somewhat, China never did come close to replacing the Soviets economically. Chinese assistance to Mongolia never exceeded 1/4th of what the USSR provided. In 1956 the Soviets withdrew their troops and the Mongolians were able to exercise greater control over their affairs.

Throughout the 1950s, China remained Mongolia's second largest trade power. In 1954 the PRC granted Inner Mongolia (内蒙古) with Autonomous Region status, and encouraged contact between Mongols on both sides of the border and setting up a consulate general in the Nei Mongol capital, as well as permitting cross-border grazing for livestock. A Soviet source claims that Mao offered Mongolia 300,000 settlers, although this has never been verified, and obviously Mongolia never took the offer seriously. This period was relatively nice for the Mongolians, who no longer had to worry about Soviet military ambitions, and was pleased with the positive outreaches from the newly founded PRC, who's original intentions were previously ambiguous.

In 1957, however, things began to heat up in Mongolia due to the Sino-Soviet split in late 1956. Mao's recently ambitious tones in Mongolia had spooked Tsendenbal, who immediately shifted gears and ran back into the Soviet camp. In 1957 a joint statement between Tsendenbal and USSR Chairman Nikolai Bulganin reaffirmed the "leading role" of the USSR in Mongolia. Soviet aid began to greatly increase. In 1958 and 59 over 6,500 Mongolian party members were purged, including First Secretary D. Damba, who was rumored to have pro-Chinese sentiments. But again, that could've just been a convenient cover for Tsendenbal to purge enemies in the state. The rejection of Chinese influence in 1957 led China to rescind most of its previous agreements with Mongolia. The Inner Mongolia AR was forced to abandon the use of Cyrillic script, and contact between the borders became strictly regulated as to cut off "local nationalism," especially during the Anti-Rightist Campaign.

As a coda, Mongolian-Chinese relations faltered greatly throughout the 1960s and 1970s. During the Cultural Revolution, Mongolians were targeted by Red Guards inside Inner Mongolia. Mongolia's state-controlled press then published several critical pieces about Chinese aggression inside the region. Soviet troops were re-stationed inside Mongolia, and when Mongolia forced its Chinese laborers to leave the country in the 1960s, it was forced to rely more heavily on USSR aid. The fall of the Soviet Union ended this unique relationship. Today, Mongolia is heavily under the sphere of Chinese influence.

My theory for the poster

Perhaps its me, but it does seem that Mongolia shares the same colors as the USSR and other states surrounding China, although there is a clear border whereas no other nation has one, signifying its significance in the minds of the Chinese. Regardless, Mongolia's border/color most likely involves the historical claim that Chinese nationalists have felt attached to. Although Mao was forced by Stalin to give up any PRC claims on Mongolia in 1950, technically the Republic of China in Taiwan still holds claim to Mongolia (since Chiang rescinded the plebiscite in 1947). And many Chinese nationalists believe that Mongolia was forced out of its hands. So there's that.

As for Japan and Korea, I would imagine that this would have to do with their alliance to the US. The Korean War had either ended, or was close to it, in 1953, so the sides in the Pacific had already been taken. What I find more interesting is that there appears to be no Taiwan. You can clearly see that they bothered to include Korea, which is small, but not Taiwan, which is the major piece of land in the Pacific front that the PRC had claim to. Regardless, this is a very interesting poster, and thank you for sharing it! You'll also notice the litany in the faces of the dancing females and men, as well as the overall lack of "red" that would come to absorb posters during the Cultural Revolution phase of propaganda.

Sources

Morris Rossabi, Modern Mongolia: From Khans to Commissars to Capitalists

Asian Survey, Vol. 26, No. 12 (Dec., 1986