I'd say leave the shields more uniform for the higher tier units, ie principes and triarii. Then have the hastati using more varied shields, I guess to simulate that their gear is a bit more "improvised"?. That's just from a preference point of view for me at least.
It looked around though and found images of it actually being the other way around (go figure, lol). So basically the hastati and principes had their normal red paint shields, while the triarii had both red, white/tan and blue. Here's the image I found of it.
Besides that all I could really find was that pretty much all polybian units had a mix of tan and red as far as shields go. The only blue I found for them was some of their tunics.
Then again I just googled all of this, so if someone actually knows, they'd probably have something more constructive.