Let's see... with 8 Tervigons and a possible 7 turns that could be 56 distinct units, so if you have an 8 by 8 combo you could cover it all. So pick the 8 Crayola box colors and paint the base edge one color and a stripe on the head another color. That would give you enough variation, but that would be more than 1000 gaunts if you wanted to cover the worst-case scenario.

A more practical approach might be to count on no squad living longer than 3 turns and going by average size, so you could do 8 base edge colors with 3 head stripe colors on 10 squads. That would only be 240 gaunts. Then you could have some with no head stripe and just the base edge colors to use as filler, maybe 8 more of each... I think 300 gaunts would probably cover it.

Why the Hive Guard? For anti-flyer fire?