Texture Painting a Node Material?

As you can see, I’ve got a nice green landscape. But it is missing some sandy patches in a riverbed. You can see that I have a nice, gritty, material titled “Sand”, and what I want is to be able to texture paint THAT material. It will only allow me to paint in specific colors, and I can’t find any tutorials on YouTube about texture painting noded materials onto a surface. Am I blind? Or is there a way to do it? This is my Node setup for my sand, if that helps.

You make a black white texture, paint it, look up its value and use that as the fac for a mix shader node that mixes between grass and sand BSDFs.

You can do this by vertex paint. Separating R G B and alpha if needed. Use this 4 channels with the same uv tiled textures for blending it. Really efficient and economical. No needs of textures.
We call that splatter.
Really old technique used before people restrict vertex color to dirt there huge consuming polygons sculpted.