Ah, ye can do that by editing the picture on GIMP. Just create a new layer, fill that layer with the desired color, and then going to the layer options (right hand corner of y'er screen there should be a layer tab) and clicking the little upside down triangle. There are quite a few options, so just sift through them to see which one works best with the color ye 'ave chosen.
This may sound confusing, so if ye want, I can provide screenies. ;3
EDIT: Unfortunatley, I do not 'ave a screen recorder (or whatever he would call it) to record m'screen. :C Terrribly sorry floof.