How do I properly use core matte to keep green section of shirt on using green screen keying? I have some footage shot with a green screen and stupid me didn't pay attention to the shirt I had on which on the center of the shirt has some green. So naturally, I have a hole in my chest when keying. My solution was to create a mask around it and then feed it to the "core matte" input on the keying node. While this does work, it changes the color of the part on my shirt.

My question is: How do I prevent color change if this is the correct way to fix it?  If it is not I'd like to know the best solution.

Thank you so much in advance for taking the time to help!
Hard to tell without seeing your setup, but my guess is :  the mask plugged in Core Matte always output a 100% value, while the rest of the shirt output a lower value ; ie, there is some green in the shirt and the keying node make it a little bit transparent. Nothing noticeable at first, but noticeable by contrast with the mask.

You can verify that by compositing the output of the Keying node on a grid, or on a vivid color. Or plug the Matte output to a Viewer, and make sure there is only black or white areas, no gray. That will make obvious if some areas aren't keyed correctly. If that's the case, adjust the parameters of the keying node.