I think your best bet is to take the upper with you to a place that sells lowers to compare.
IMO, every "brand" of lower has had differences in color even compared with there own. I have bought a few complete factory built Bushy's with mismatched color, then traded around upper/lowers among them and still couldn't get a "perfect" match;and have built about 5 rifles on Grizzly lowers---the first 4 were pretty damn close to the same---then the last one I built about 2 weeks ago had a MUCH different color---didn't really notice or compare when i bought the lower 2 months ago.
Probably has more to do with variations in the anodizing vendor/batches than anything.
my 2 cents
ETA: not much of a Colt fan, but the earlier (Colt grey) rifles ALWAYS seemed to match, even swapping around--much more consistent