i think it is more likely to be a variant in the species as opposed to a cross breed, i have the polypterus delhezi, supposedly an off white base color with black markings but sort of similar to how you have described the endlicheri, which is what i assume you are talking about, with yellow dots. well it is more like a yellow outline to the black markings on it, its not just yellow, its like a florescent yellow! with seeing that on mine i would assume it is quite possible for other species of polypterus to attain slight color variations. the darker color i would put to maybe the fish being unhappy or stressed, if you have seen it in a shop then it more than likely will be put there to be sold as opposed to all its needs being met.
David
