What color for tunic

[Caius Fabius]

What unit are you trying to re-enact with? That is a modern day question that you should answer, when picking a tunic color. Dan Peterson's XIIII GMV has a red brown wool almost the color of dried blood. Other groups use a dark wine red, from madder dye swatches. Other groups run a gauntlet of reds to pinks! There are other colors, Julius Caesar's LEG V the Larks may have used yellow tunics, and there is an ongoing discussion of auxiliarry colors. Crimson was probably reserved for the higher rank, possibly centurions and richer officers? Julius Caesar's cloak was crimson and was distinctive on the battlefield. There are loads of answers, which include white or natural wool tunics, we know the centurions at one time petitioned to be allowed to wear white tunics in a triumph, was that so they could look like the rest of their men, or so they would look different?
Ongoing experiments with washing the wool tunics in ammonia show that the same ammount of effort goes into cleaning a red or a white tunic, but the red-brown dried blood colors or the deeper red wine colors seem to show dirth and metal deposits less than either white or red. There are no color photographs of Roman legions in the field, and conetmporary artwork did not retain the colors that were painted on the sculptures of Romans at war.
That being said, you are only going to be WRONG if your tunic color stands out from whatever unit you fall in with at events. If you are a lone re-enactor you will probably need a couple of woolen tunics in different colors.