Limitations are what you make of them. Believe me, on 15mm figures distinctions of coat colors get very important to keep from getting confused - especially in the early 18th century when, for instance, the Dutch and French armies both had troops wearing coats colored red, blue, white and gray and they were the primciple opponents for most of the wars of the early 18th century!
That, in fact, is how I ended up with so many different shades of red and blue: looking for distinctions among them.
Purple is perfectly useable, as long as you dont't use 'true' purple until after 1860. Purple was used to describe mixtures of blue and red that we would call Maroon or Dull Red, and the fact that most of them would wash right out of the fabric in a light rain we can ignore in the game. Similarly, green was used by a few armies in Europe, but they quickly made it a very dark green, because all the green pigments would fade and 'gray out' in sunlight - none were 'light fast' and in modern museums in France and Russia they are kept behind glass cases in rooms with no access to direct sunlight at all.
And among the most common fabric dyes before modern chemical pigments, 'Red' can be anything from a dull brick red to a bright crimson, but the latter is for Royalty (or at least, Army Commanders) and other 'special' types. Blue can be blue-gray, dark blue, medium blue, light blue, and even shades of blue-green depending on which of several different vegetable dyes or combinations are being used. One advantage of Indigo (native to India, introduced to Europe in the mid-17th century, so Exploration Age) is that the fabric gets darker the more times you dip it in a vat of Indigo dye, so that everything from a very light, 'sky' blue to a very dark 'royal' blue can be obtained from the same plant and dye.
By my count, just among red, blue and mixtures of the two that gives us a minimum of 8 distinct colors without even considering less-common shades like orange and yellow, which were possible but, like green, transient in 'real life' but useable in-game without completely divorcing the graphic from any reality.