I think the best answer for you is to learn how shaders work. A few weeks ago, I was like you, understanding nothing about shaders. But thanks to Megalomaniak on the forum, I could learn the basics of shaders with those tutorial videos: https://www.youtube.com/playlist?list=PLbPC2AiSSX7_TSiVxlqKon_LL2Rr1z_3P
Note: the tutorial is for UDK and not Godot, but it's really almost the same. You just need to find the equivalent nodes and you get the same result. The theory is the same, and that's what matter.
When you're done with, let say the 2 first videos, you'll see that you just need to append a Vector of color white to the input.