the abs they use is great stuff personally i would keep white as its acrylic capped. now the matte hips scootch uses i would paint since its already a little rough. anyway though, make it fit then paint it. you dont have to do the inside and i would avoid getting any on the inside for the sake of not adhering thus flaking paint all over your undersuit.
srsly though its going to be a lot of work. to prep the surface wetsand with around 400 grit, preferably with a random orbital sander to make things faster, and a sanding sponge for the smaller recessed and curved areas. then theres 2 options, spray the krylon paint+primer then clear it and polish after dry, or use a thin coat of cheap primer but spray rustoleum professional enamel as a base. i used rustoleum for my helmet but without clear. if you clear it lay at least 3 coats, but its glossy and durable enough where i felt it wasnt needed. like i said you might just want to hold off and stay white.
another option is using vinyl wrap, like advertisements on cars and window decals. you can get black in bulk and will survive trooping easier, plus if it tears its easy peel off and replace. cheaper and less work than paint as you dont need to sand or anything, but you have to be careful about air bubbles, so use a heat gun to help form around detailed areas.