Fluff-wise, have them represent an ex-imperial army which has totally fallen to the Cult - that gives you the excuse for their clothing to all be the same as it's simply their old uniforms (far more believable than for individualistic orks

). The obvious exceptions would be banners and insignia added post-cult to proudly proclaim their new allegiance.
With 8 sets, you'll obviously have a lot of repeat figures, so look at easy ways to customize them - head/arm/weapon swaps, extra equipment etc. Paint can help too, with stains and general dirt (laundry is not a priority for the
GSC).
Yellow and purple can be horrible colors to get good solid coverage with, although that will depend on the specific paint used, so be sure to do a few test figures before finalizing on those colors.