Welcome aboard! Unfortunately the only plastic liines of
IG infantry are Cadians and Catachans.The new command boxes are quite nice, whereas the normal catachans look like human apes.

There are however the resin Elysian Drop troops and
DKOK. In metal you can find Vostroyans and if you' re lucky Mordians.
Do not despair however because i have seen kit bashes with the lines of fantasy. Empire Flagelants and bretonian Men-at Arms have nice bodies that are compatible with the cadian plastics with a little effort.
To the painting: don't bother withe the choise of colours for your Guardsmen. Pick a quick and easy to reproduce colour scheme that looks good to your eye and go for it. In my army BS4 models have Vostroyan and Stormtrooper models and are painted with blue Cloth and red Armour while BS3 models are cadian plastics with Kommando Khakki cloth and a very dark grey armour.
Hope i Helped