Start with a model with pale eyes for most dramatic effect. Isolate the eyes by setting a black background layer, adding a mask to your model layer, then erasing and obscuring other facial features. This is achieved through simple adjustment layers and masks.
Apply a Black & White layer, tweak settings to give skin a burnt effect, setting an 80% Multiply blending mode. Tweak skin colour and saturation with Hue/Saturation. Now mask away Black & White effects from almost all of the eye, except the edges of the iris.
Next create a Colorized Hue/Saturation layer, choosing your favourite tone, place above your model layer, invert the mask and paint in colour effects. Animal eyes are as simple as importing stock, masking the eye detail and setting a Luminosity blending mode.