I'll give examples of what I'm talking about (sorry they were from
Stratics, so the images are small, but hopefully you'll be able to see what I'm talking about):
Pure Opifex Female
Notice the pure whiteish gray skin, as though the face were carved from marble. The hair is white showing that pure breed Opifexes are probably naturally devoid of pigmentation of any kind due to their genetic profile. While some Opifex have colored hair, this was probably added later as a cosmetic touch.
White Opifex Female
Notice the slight pinkish tone to the skin. Also, bangs indicate naturally colored hair follicles even though hair in the back is dyed orange for cosmetic preference.
Asian Opifex Female
Notice the pinkish brown tone of the skin, coupled with the slight alteration in eye shape which is indicative of an Asian genetic heritage. Also, note that the hair is black which also harkens back to the idea that this Opifex was blended with Asian genetic markers. Further, the face has wispy eyebrows which are common among Asiatic people.
Black Opifex Female
Finally, notice the brownish gray tint to the skin and larger eyes which seem characteristic of an Earth African background. Again, hair is dark, and eyebrows are thicker than the other faces represented here. Lips are just barely fuller than the other models.
So, there absolutely IS a difference in genetic background for each of these Opifex models. It is probably the case that Opifex interbreed more commonly with Solitus which would lead to a wider variety of skin and facial features than are common for Nanomage (who may only be able to mate with other Nanomages) or Atrox (who, of course, cannot mate at all).