Spir Aabye

Forgive me if this should be basic knowledge, but I am returning to the game after not playing for years and forgot some things about editing. I attempted to darken one section of an item, and the whole thing turned one solid color. Trying to fix it, I could only change the color of the whole item, not the individual part of it. So I feel kind of silly asking this, but I spent way too much time attempting to figure it out on my own, but how do I select a specific part of an item without selecting the whole thing when I edit it?