Hello lokiie, I've just learnt how to do something similar myself, so maybe it will work for you.
1. Click "Select by Colour" tool
2. In the options window below, change Threshold to about 90
3. Click somewhere green in your image
4. Create a new transparent layer and call it 'Colour Overlay'.
5. Select a deserty colour
6. Drag the colour across to your marching ants
7. In the Layers window, click Mode and select Colour.
8. Play around with different colours in steps 5 and 6 until you're happy, then right click on 'Colour Overlay' in the layers window and click Merge Down.
There might be other easier ways to do it, and I'd be interested to find out if so.