Colorize typically maps a range of luminosity to a black-to-color-to-white gradient (in other words, its maximum effect is on midtones). In your image there is only black, so it remains black, unless you push the lightness a lot but this will still give you something a bit darker than your color.
If you want to replace the black by another plain color, just alpha-lock the layer (checkerboard icon in the
Lock: line at the top of the layers list), and bucket-fill with the required color: due to alpha-lock pixels will keep their opacity/transparency.
Colorize won't work on a totally black image unless you adjust the Lightness slider. If you set it to 0.5 it should match the sampled colour.
Anyway, If you want to colour that image with a sampled colour, I'd employ a different technique that avoids messing with sliders. The following will work with any solid image with transparency, regardless of colour.
- Right click the image layer and choose Alpha to Selection, or engage the Alpha Lock on the layer.
- Choose a new foreground colour
- Do Edit > Fill with FG colour.
Or, if you want to get creative, paint over the graphic with various colours/brushes. The selection or alpha lock will allow you to only paint on the solid pixels.
You can add a layer, filling it with the desired color or pattern and use Gimp Layer Modes to obtain the desired result (Lighten Only should work).
Starting from your image (I've borrowed it from your screenshot, so the contour is a little pixelated):
You can add a layer with the desired color and set "Lighten Only" (your image is black) as blending mode:
You can also use a different image in the new layer, a gradient for example:
The resulting image: