Yes, that's how you load custom textures, but no need to press "null" if you gonna replace it anyway.
Use "null" on stuff you don't wanna (example builtin spectex, glosstex, bumbmap) otherwise "clear" option works on non-builtin stuff.
If you set specular int. at "0" or SpecTex to "null" than extra colorization/mask should disappear.
Worth looking into global illumination filter setting (or scene lighting illum master intensity)
it could cause extra color shift (also make sure iris color is white and diffuse offset at 0).
If something looks cut or not centered you can use X/Y offsets to try aligning image or tile X/Y to change size.
That example image for iris/screla is bit low res (512x512) maybe try more highres one 2-4k?
After overlaying UV over it, it looks a bit of center to me? but i am not texture expert