Colour of ancient clothing by Forgus
I made a little reseach on the topic, here are the much simplified results:
Fabric colours in antiquity
Blue: Basicly two blue dyes are available in our timeframe, woad and indigo.
woad is available in Northern/mid Europe, while indigo comes from India.
They both contain the same dye, though indigo a lot more. Depending on the number of washes all shades from light blue to dark blue could be acchieved.
Takeaway: Blue dye is not very expensive, even in darker hues
woad varieties
indigoes
Red: The primary source of red in Europe is Madder. It produces a colours from pink to rusty brown, including duller oranges. It was available all over Europe, and came quite cheap. Archil too was used in the Mediterranean, that yields a purplish colour, but is more expensive
Takeway: Red is quite cheap too.
Madder varieties (ignore woad)
Yellow: yellow can be produced from a number of plants, like safflower, chaomime, or peppergrass, these gave duller colours. Brighter yellow came from weld (could be a bit greenish) or saffron full, warm yellow. These came much more expensive, especially saffron.
Takeaway: easy with bright yellows, use on officers, or make your colours desaturated
wool painted with weld
Buddhist monk in saffron coloured robe
Green: Green dye was not available inour timeframe, though good results could be acchieved by overdying woad with weld. Also, weld treated with copper should be light greenish.
Takeaway: Beware Epirotes, geen dye is expensive!!!
woad on weld
Purple: you all know about tyrian purple, but cheaper blue purples are available via overdying madder with woad!
Takeaway: Blue-purple is not off limits, especially not for officers!
Tyrian purple
Black: There is no black dye in antiquity, however similar results could be acchieved using wallnut iron, or heavy indigoing. Other recipees are available too, but the result isn't long lasting.
Takeaway: Rich black is expensive, dark gray is more likely, especially on a soldier.
White: Not dying, but bleaching was used frequently, moderately expensive.