Gimp: changing all shades of a colour to another colour

Question

Have an image which is (basically) greyscale, white on black. I want this to be green on black instead.

This answer seems close, but I cannot get it to work. I probably do not understand steps five ("Make a new file with only the color you need") and six ("Make a Layer mask"; not sure which options to use, and there are no "channels" available).

I understand I can manually repace each individual shade, which does seem practical but would take some hours. I'd rather do it in one operation.

Answer 1

1. Create a new layer, fill it green (use the bucket fill tool)
2. Move the green layer under the black and white layer
3. Set the black and white layer blending mode to Multiply

Example

Answer 2

A technique that gives good control is Color > Map > Gradient map. It replaces a pixel color by a color sampled in a gradient using the pixel luminosity. So for instance applying this gradient:

to this image:

produces this:

In the simple cases you can use a black-to-color gradient

Or a color-to-white gradient: