Well this is really easy in paint.net
First go in paint net open your picture.
Second click adjustment in there click curves
Third click on Luminosity and change it to RGB
4th check the box at the bottom of the colour you want
5th click in the curves box and move the point until you have the colour you want if you can;t get it in curves, go to adjustments, Hue/Saturation if you want a darker colour carry the saturation up if you want a lighter one thats not in curves carry down the saturation
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