As you can see by this photo:
http://i.imgur.com/pdA8kGu.png

The BuildCraft tanks apply the colour properly, but the RailCraft one doesn't seem to do it.

(Also, the texture of the fluid seems to be scaled by 2 in the RC tank. It's been like that for a while so I'm guessing it's an intended feature, but I thought I'd let you know just in case it escaped you)

Hell if I know, I can't even seem to force a color change by hard coding a GL color call.

Ok, figured it out.

Hmm...the code exists to do so, perhaps the gl call is being replaced.