Müller's great discovery concerned the resemblance between two or more unpalatable species which are protected from predators capable of learning. The protection is often a noxious chemical, perhaps gained from the larva eating a particulat plant; or it may be a sting or other defence. It is an advantage for such potential prey to advertise their status in a way clearly perceptible to their predators; this is called aposematic or warning coloration. The principle is of wide application, but in Muller's case the prey were butterflies, and the predators usually birds or reptiles. The aposematic colours are most often some combination of red, yellow, black, white, whereas palatable animals are usually cryptic.
(Wikipedia)

In 1879, Müller used his flair for mathematics to show that one unpalatable, warningly colored species would benefit from resemblance to another unpalatable species by a factor equal to the square of the inverse ratio of the species' relative abundances (James Mallet, Galton Laboratory, University College London.