The items themselves render right, but when used to make a tool they turn white instead of retaining their colors.

Which version are you using and what other mods do you have installed?

Mod version 1.1.5a, forge version 40.2.0
After a little testing, it seems to occur when Rubidium is present

After looking into it there's nothing that can be done on my end, it's an issue caused by a bug in rubidium, that seems to be affecting quite a few other tinkers addons as well as some other mods.

Did a little digging, looks like there is a workaround:
Asek3/Rubidium#166 (comment)