Main thing about a Mountain bike is top tube clearance. At least 4" clearance between the crotch and the top tube is the suggested but That is only important if You are a rider that will suddenly jump off the saddle and put both feet on the ground. I normally hit the ground by falling off and as the bike is horizontal - or vertical inverted when that happens- It doesn' t bother me too much.

Colour doesn't matter on a mountain bike as it soon finishes up as mud tinted.

Sizing-I ride a 40cm compact mountain bike and a 51 road bike- or 46.5 if it is compact frame- But there are so many variables on set up with long seat posts- riser bars and bar ends- It is not critical. Just get more than 2" clearance and you will be fine.