I've been researching how to brighten the iris and change texture in depth for the past couple of days (and a couple years ago) and it is needless to say it is highly difficult.

The eyes (specifically the irises) are different in melanin expression compared to the rest of the body because nearly all of the melanin exists in intracellular deposits and there is a lack of melanogenesis (though a small amount), instead the existing melanin deposits largely created when very young are static and don't change much.

There are ways to reduce melanin on skin, for example, by reducing creation of new melanin as it is replaced. However, that won't work for the iris as the existing melanin won't change. If you look into genes which modulate eye color (OCA2, HPS-5/6), nearly all of them can't be targeted directly as they are genes and not druggable targets, and even if you can find ligands which target related pathways to these genes they are unlikely to change adult morphology due to the maturation of melanosomes.

To change eye color from brown or brown-hazel to a lighter shade such as blue/green, the melanin deposits in the frontal iris epithelia need to be degraded. One theory I have is that if a melanosome modulator inhibits dendritic length and frequency while effecting translocation and then you combine that on top of a melanocyte autophagy and tyrosinase inhibition which is penetrant intra-ocular then effective melanin reduction could be achieved.

The main limiter to achieving iris lightening is reducing the existing intracellular melanin deposits. Extracellular reduction is easier but not relevant. If intracellular reduction can be achieved then it is very likely that rayleigh scattering could be induced leading to lighter eyes.

I have a couple compounds I'm looking at that are mostly untested but have a decent potential, I'll test them in one eye (after doing more research to make sure I don't go blind).