Adobe Photoshop is the most commonly recommended choice if you want the most capable tool for this kind of edit. It gives you precise control for selective color changes like adjusting iris color.

If you want a free alternative, GIMP can do similar eye-color edits as well.

For heavier portrait-retouching workflows, a dedicated portrait editor such as Portrait Professional may also be worth considering, especially if you plan to do this type of adjustment often.

So in short:

• Best overall: Adobe Photoshop
• Free option: GIMP
• Portrait-focused option: Portrait Professional

Whichever editor you choose, the usual approach is to make a local adjustment to just the iris rather than changing the whole image.