The blue node on the right is a Math Node - it can only output single channel values between 0 and 1 (opposed to three channel RGB values) so of course the result is not colored when it’s the last Node in the chain. You probably want your last Node to be a MixRGB Node - perhaps the one on the right next to the Math Node?
Also, giving the DiffuseBSDF a color won’t do anything too because as soon as you feed it into a Math Node, it’s converted to a grey value. In general, grey Sockets represent Sockets that can only take/output single channel values. Yellow ones are for color (three channel RGB values)
Anyways, the easiest way to achieve what you probably want is something like this: