If you mean to paint a 'Brush' on an individual triangle, you would need to assign that triangle to a surface which has been painted with that brush.
If you want to paint new detail onto a texture map, you would need to find the UV position on Texture map and write to the TextureBuffer() at the specified point.
If you want to paint new detail onto a texture map, you would need to find the UV position on Texture map and write to the TextureBuffer() at the specified point.