To change colour convincingly you really need new eye textures - most characters come with several and in general you can mix and match between character sets (obviously a painterly or toon style set won’t look good with eyes from a photo-real set, or vice versa, but as long as the over all style is similar they should work). However, you could try adjusting the diffuse colour of the iris material to tint the eye -especially if there was a fairly neutral grey eye colour to start from