Then I use the brush tool to get this little area over here. If I hit X on my keyboard, it will toggle the foreground and background colors. I'll reduce the brush size. I just want to hit get this little corner area. Set that to normal, most people are going to have that set to normal. Now, when I'm done painting inside of here, if I hit Option again on my keyboard it returns me to the original image. Now what I'm going to do is I'm going to add an adjustment layer to remove the yellow. So I'll start by clicking down here on the Adjustment Layer button, and I'm going to go to Curves.
Now, what I want to do is I want to change the color balance on the Copy layer with the teeth on it, but I want it to only affect the layer with the teeth. So if I hover my mouse over the border between the Curves layer and the Copy layer and hold down Option on my keyboard, you'll see that the icon there changes. Once it changes, I'll click, and what that will do is it will attach the Curves layer to the Copy layer. Now anything that I do in the Curves parameters is only going to affect the Copy. The way that the Curves work is that this control here, this slider, control the highlights, and this slider controls the shadows. If I click in the middle, that will create new key frame and it will control the mid-tones. So I can change the color balance of the image using the Curves slider.
Right now I'm working on the RGB layer. So anything I do is going to change the color mix of all the pixels. Yellow tends to be a mixture in RGB of reds and greens. So I'm going to start off by going to the reds, and I'm going to reduce the red balance on that layer. Next what I'll do is I'll go to the greens, and I'll reduce the green balance. I'll take some of the greens out. Then I'll return to RGB, and maybe I'll just push the highlights up a little bit. If I zoom out, you can see that her teeth are already whiter. If I just turn off the layer, you can see the color difference right there. That's how you remove yellow from teeth in Photoshop.