I have a texture which contains a number, like in the picture below. And I want to 'paste' it over the tshirt texture. I have tried using mixRGB but I have this effect in which you can see the back stripes in the number. And I would like to have it straight white. Additionally, I would like to add a node to control the color of the number. If I change the mixRGB node mode to lighten I can have the desired effect for the white numbers. But then I can't change the colors correctly.
To add to Chris' answer, you can do it this way, mix the 2 textures into a MixRGB, use the alpha output of your number as factor, and choose the color you want in the MixRGB. You can use another UV Map before the number texture if needed. It works fine if you don't need to change the Principled BSDF settings of the number (like its glossiness), otherwise Chris' answer is more convenient: