I know this is an extremely basic question, but what is the simplest way to add color/tint a TOP texture? (Besides changing the color in the rectangle/circle top.)
The lookup TOP seems like the operator to use, but here it just recolors everything:
A lookup table is modifying input colors based on a gradient. Basically : change black to this color, change grey to this color, change white to this color and so on…
On your example, every gradient of your input image is colored with a unique color, so everything ends the same color. If you need to change only white, you need to use a ramp and change the white part of the ramp to the color you want.
Though for a simple color change like you mention, you can use compositing : use multiply blend mode between the color and the square.