I'm pretty sure that the red was originally added for aesthic purposes only. It became traditional, at least in the regiments, just because that's what "we've always done."

I have found that many bonnets have a same-colored toorie rather than the red. I purposely bought my first bonnet with the black toorie rather than the red one; picked it out of the lot intertionally.

You can also make one very easily by following instructions printed in So You're Going to Wear the Kilt. All you really need is some yarn in the right color.