I'm a little late but essentially because of how warfare worked at the time the armies would be recruited on campaign and most if not all soldiers except officers were militia or mercenaries and typically would just be wearing whatever they had while also being issued whatever the officers were carrying with them such as buffcoats and cuirassies... the only nation to actually standardized uniforms was Sweden as they were the only real "professional army" of the time... however there were some common things that soldiers from certain nations would try to wear to be somewhat uniform... for instance alot of imperials wore red stockings and alot of French wore blue or green saches... another reason is the rate at which men died in the period... war was incredibly deadly and outfitting men with expensive uniforms just wanst viable... however exceptions did exist because each captain or general would have to recruit and outfit his own regiment with his private funds... so there were cases in which regiments had standard uniforms but as I have stated this was rare... hope I helped clear things up!

u/walren_emperor45 in right in regards to the militia. With exception to that of Wurttemberg, they were generally under equipped, not just in regards to cloth but armor and pikes as well.

However, this wasn’t the case for mercenaries, especially in the early part of the war. Vibrant, bright colors such as red, blue and green were very much associated as “colors fit for a soldier,” and as such captains and colonels would spend great sums of money to outfit their companies or regiments in such uniforms. In a few documented cases, the officer would buy one color for the entire unit. Indeed, being provided new clothes each campaign was certainly an attractive prospect for mercenaries who were regularly employed.

The early Protestant armies before the Danish intervention were the exception to this rule, as their armies were made to be cheap to raise and more to hold ground than fight on the field. However, even then, at the Battle of White Mountain an Imperial Source claims that the Protestant forces used sky-blue, but this likely refers to the field symbols rather than uniforms.

As a side note, “mercenary” in its 17th century context is much different from how it’s used today. Contemporary mercenaries, especially infantry, were very often personally involved in the conflict, and were integrated into the militia ties of the different European powers.

Another thing to note is that Sweden was very far from a professional army. While Sweden drafted its men for combat, this was largely a case only for the infantry. The cavalry would’ve mostly been volunteers who wanted to avoid being in the infantry. However, the fact that Sweden had a permanent army doesn’t make it any less “professional” than that of mercenary-heavy armies, whose soldiers, especially as attrition began to set in, were just as skilled if not more skilled than that of Sweden. On that note, do keep in mind that the vast majority of Swedish troops were still mercenaries, as with most other armies. Indeed, the most famous uniformed regiments of the Swedish army during the war were German, not Swedish. There is some evidence for blue being the uniform for native regiments, but solid proof doesn’t come until it was implemented across the board for the entire army. Do note as well, Wurttemberg’s landmiliz had a well-funded, well trained infantry arm that outnumbered the men in native Swedish regiments in Germany at the time both were active.