Could I possibly get a step-by-step guide for using Gradient maps? I've tried using it myself but whenever I go to pick my colors it changes it to the greyscale equivalent (ie, picking light blue gives me a light grey, picking dark blue gives me a dark grey). Is there a certain layer type or spec I need to specify first or am I just going about the process wrong? Any help would be appreciated.
The gradient map takes the shades of grey, from your image, as the input then outputs the colour set in your gradient.
So if I take this image of yours truly :
Add the gradient map - as an adjustment layer
Set the gradient to run from black through red to blue
Then the output looks like this The darkest areas are mapped to black in my gradient the lightest to blue
If I click and add more points into the gradient (clicking on the colour square to change the colour at each point)
I get this
One other comment - you need your document to be in RGB /CMYK/Lab mode to see the colors. If you are in grayscale mode then grey is what you will get