i have some images of greyscale pixel art tanks and ive been trying to colorize them, using the colorize tool gives very weird results as it is intended for photos not art. the orginal image is this 

i want to colorize it and get a result like this:

if im misusing colorize or there are some settings i could change to get it to work please let me know

Here you go.

Select the tank bitmap and [Path > Trace Bitmap...] with the default options. Combined the paths into a tank outline and fill the resulting shape blue. Lower this shape below the bitmap. Select the bitmap and open the Fill and Stroke panel [shift+ctrl+f]. Set [Blend mode: Multiply]. Group the bitmap and the filled outline. You can change the tank colour by changing the fill colour beneath it.