How can I convert a coloured icon to a monochrome one?

Question

I recently compiled goldendict from source. To my dismay I discovered that there were no monochrome appindicator icons included. At the moment I am stuck with this:

I tried converting the icons to B/W using GIMP, but it still doesn't look quite right:

How can I modify the icons to fit with the rest of the theme?

These are the icons in question.

Answer 1

Ok, I invested some time and did it myself. What I did was basically use GIMP's Alpha to Selection tool to select the non-transparent parts of the image. Then I chose the Bucket Fill tool to apply a homogenous grey colour to the icon. All other refinements I did manually with the Paintbrush tool.

Here's the result:

You can download the icons here.