Indeed missing walking legs is a common occurrence and if in a young nymph (at L2 or L3) should repair back to normal before reaching an adult - if it is closer to an adult it may only have time to grow a partial leg if it all. I had a mantid nymph from L2 that only had one raptorial forearm/front leg it's entire life, and never tried to repair it; although, any that I had with damaged or missing walking legs repaired it if they had enough time before adulthood. As adults they can not repair it as they no longer molt.
Regarding the color it is likely normal, as the coloration can widely vary from individuals even in most species.
For example in Carolina mantids I've kept (Stagmomantis carolina) I've had them range in color from brown, tan, black, light green, dark green, and even a almost neon green. In some they also had unique coloring banding/camouflage as well - it seems their coloration varies on their environment (and the widest varieties of colors are from personally wild caught ones).
If you can post a photo or a link to one (or several) that will help to give definite answers to your questions.
