How do I change brush-marks of a certain color (everything that is not white) into another color. For instance, having lasso-selected the following brush-marks, how do I change the color from black to red?
Set the color mode of the image to RGB. Insert 2 adjustment layers. On the top you should have layer Hue&Saturation, option colorize=ON to set the color. Below it you have layer "Curves" to lift the black to grey.
The colorization is disabled in the next image, the curves layer lifts black to grey:
The full effect:
Flatten the image or merge these 3 layers to fix the effect if needed.
There's no need to use adjustment layers. You can as well apply at first Image > Adjustments > Curves to change black to grey. Then apply Image > Adjustments > Hue&Saturation > Colorize to get the wanted color.
Making grey is a must, black and white cannot be colorized.
Another option is to invert your image to have white text on black and use it as a layer mask to solid red layer.
The third option is to make the black text to grey and insert to top a solid color layer with blending mode = Color
Methods 2 and 3 allow non-uniform coloring like this
Method 2 only allows other than white background. You must insert a new BG layer because layer mask makes the original BG transparent.
You can edit the layer mask onscreen if you Alt+Click the layer mask icon in the layers panel. This is needed for ex. to paste your text to the mask.