Might I humbly suggest
https://www.keepyourpowderdry.co.uk/2018/06/what-colours-to-use.html for a guide to colours? I have tried to link to gather lots of links, references to paintings/items in museums and I've recently added colour swatches (for want of a better word) of colours available in the C17th, courtesy of a re-enactor.
Good thing about ECW is that uniforms were pretty generic. You can't really go wrong with a red coated or blue coated regiment for either side.
My blog is devoted to gaming the ECW in 15mm using PP figures. If nothing else you can laugh and point at my painting and say "I can do much better than those". The sculpts lend themselves very nicely to having a wash applied. There are some absolutely brilliant painted examples over on the Pendraken forum, in the not Pendraken bit.
Basing - what would I know, I base somewhat esoterically. 40x40 for 8 foot would physically fit (I base 4 foot on 40x15); horse might be a bit of a push physically fitting them in the space, mine are 4 horse on 60x30.