While Marx and other 19th Century thinkers influenced by him had things to say about imperialism, it wasn't a major part of their thought and they largely did not see their political project as useful for non-Western societies who were trying to resist British, French or other imperial powers. (19th Century Latin America is a really messy and complicated history to try and fit into that frame and almost requires yet another analysis.) To some extent, you have to remember what imperialism looked like to Marx in the key years of the development of his thought, culminating with the publication of Das Kapital in 1867. For one, this is the period where to some extent we are in between the "old" mercantilist empires and the "new imperialism" that intensified dramatically in the 1880s. But there were still imperial wars and conflicts in this period--the Anglo-Xhosa Wars, the Anglo-Asante Wars, the "Indian Mutiny" and so on. For at least some of those conflicts Marx or almost any other European intellectual of any political loyalty was ill-positioned to have sufficient knowledge of the non-Western societies fighting against the expansion of European authority or to show solidarity and support for their struggle. But the Indian Mutiny is fairly instructive here about the potential for anti-colonialism even in Marx's own thinking, as both Marx and Engels viewed it not as imperial propaganda would suggest, a narrow mutiny of dissatisfied Indian soldiers, but as a broad social uprising. Marx wrote in 1857, " the mutiny, has not been confined to a few localities; and lastly, that the revolt in the Anglo-Indian army has coincided with a general disaffection exhibited against English supremacy on the part of the great. Asiatic nations, the revolt of the Bengal army being, beyond doubt, intimately connected with the Persian and Chinese wars". While he didn't have a tremendously sophisticated grasp of the specifics of South Asian history, his sense that the societies of South and East Asia had revolutionary potential and were contesting the authority of European capitalist empires was fairly plain. But he also essentially viewed imperialism as a necessary engine of capitalist modernity, which he thought in turn was a necessary precursor to the overthrow of capitalism--a position that most 20th Century Marxists eventually rejected or sidestepped.

Imperialism became a more specific part of Marxist thought substantially as a result of writing by Marxists just before and into World War I, including Nikolai Bukharin and Rosa Luxemburg. Probably the best known of these writings is V.I. Lenin's 1917 book Imperialism, The Highest Stage of Capitalism However, it's important to understand that the book was anti-imperial rather than being anti-colonial, and even then only in a complicated sort of way. That may sound like a very academic distinction but for the OP's question it's important. Lenin was very influenced by the British classical liberal J.A. Hobson's attacks on the South African War (aka the Boer War). Hobson argued essentially that the war was a betrayal of liberal capitalism--that it was an expensive, destructive, and unnecessary folly that sought to force connections that were better developed naturally through trade and competitive advantage. While Lenin was obviously not in agreement with the idea that capitalism could globalize perfectly well without imperialism, he was inspired by Hobson's insight that imperialism was unnecessary and asked, "Well, why are the most advanced capitalist countries so engaged by imperial expansion?" His answer was that imperialism was a sign of a crisis of overaccumulation in the capitalist core economies and that it was a way to try and delay or defer the collapse that this crisis would shortly produce. (Which for him and other Marxists seemed to come true with the start of World War I.) But Lenin was not particularly interested in the struggles of colonized peoples, nor did he see them as revolutionary subjects in their own right. Imperialism was a diagnostic of capitalism's coming collapse, but colonial societies would through their own political opposition to imperial rule would not cause that collapse.

In the interwar period, however, as anti-colonial nationalism began to develop in East Asia, Southeast Asia, South Asia, the Middle East and Africa, a substantial number of colonial intellectuals began to take a strong interest in Marxism both as a source of analytic insight into the history of the last fifty years and as a way to mobilize resistance to imperial rule. (Something similar happened in Latin America, but with the exception of Cuba, this was largely about revolutionary movements directed at local national governments rather than imperial occupiers.) The Bolshevik regime in the Soviet Union also became much more attuned to the political possibilities of anticolonial revolt.

I'll use the example I know best, which is that the South African Communist Party was founded in 1921 and then was heavily involved in organizing a major revolt of miners in Johannesburg that began in December 1921--but this early period, it was a white supremacist party, e.g., it argued that only white men should be seen as revolutionary workers. But by 1928, the party, in dialogue with the Communist International, shifted 180 degrees to seeing the goal of Marxist organizing being the liberation of all South Africans and the establishment of a "Native Republic" ruled by the black majority. From that point on the SACP was led by Africans and was clearly focused on the overthrow of white minority rule.

The left in the US and Western Europe didn't fully embrace anti-colonialism as a key component of leftist thought until the 1950s and 1960s, first in encouraging socialist parties and leaders during decolonization (and in criticizing efforts by France and England to resist decolonization, especially MauMau and Algeria) and then in seeking solidarity with anti-colonial movements that became antagonists of the United States (as in Vietnam) or that were fighting in the remaining holdouts against decolonization (Angola, Mozambique, Rhodesia, South Africa.) I think you could say that anti-colonialism only became a fully orthodox part of leftist thought in the 1950s and 1960s, even if it had a deeper history, especially among colonial intellectuals and in social movements in the non-West in the 1920s, 1930s and 1940s.

You may be interested in my answer here which addresses conflicting visions of history from classical Marxism and from anti-colonial Latin American thinkers.