GIMP Tutorial #021
Going from this: 
to this:
1. Open your base and duplicate it.
2. Set this layer to ‘Screen’ at 100% opacity. Go Colours>Curves. Plot these points:
Value: 87, 142
Red: 36, 21 / 223, 230
Green: 113, 102
Blue: 197, 230 / 39, 27
4. Duplicate this layer and set it to ‘Addition’ at 20%.
5. Duplicate it again and set it to ‘Value’ at 60%. Go Components>Colours>Channel Mixer:
Red: 130, -15, 5
Green: 0, 120, 0
Blue: 0, 50, 60
Uncheck Preserve Luminosity.
6. Duplicate this layer and set it to ‘Normal’ at 50%. Go Components>Colours>Channel Mixer:
Red: 105, -10, -20
Green: 3, 92, -3
Blue: -3, -5, 98
Uncheck Preserve Luminosity.
7. Duplicate this layer and set it to ‘Dodge’ at 10%. Go Image>Merge Visible Layers>Expanded as necessary.
8. Go Colours>Components>Channel Mixer:
Red: 98,0, 3
Green: 0, 100, 0
Blue: 0, 0, 85
Uncheck Preserve Luminosity.
9. Go Colours>Colour Balance:
Midtones: -15, 7, 23
Check Preserve Luminosity.
10. Create a new layer filled with 130a64. Set it to ‘Screen’ at 30%.
11. Create a new layer filled with ff0000. Set it to ‘Overlay’ at 20%.
12. Create a new white layer. Set it to ‘Grain extract’ at 20% and add layer mask. Copy and paste your base into the mask. Invert (Colours>Invert) the mask. Merge your layers again.
13. Go Colours>Components>Channel Mixer:
Red: 120, -30, 10
Green: -10, 110, 10
Blue: -15, -10, 140
Uncheck Preserve Luminosity.
14. Duplicate this layer. Go Colours>Auto>Colour enhance. Set this layer to ‘Colour’ at 30%. Merge your layers.
15. Duplicate your base. Go Colours>Colourize. Use these settings:
Hue: 126
Saturation: 10
Lightness: 10
16. Invert this layer and set it to ‘Subtract’ at 10%.
17. Merge your layers and sharpen (Filters>Enhance>Sharpen).
There you go!
to this:
1. Open your base and duplicate it.
2. Set this layer to ‘Screen’ at 100% opacity. Go Colours>Curves. Plot these points:
Value: 87, 142
Red: 36, 21 / 223, 230
Green: 113, 102
Blue: 197, 230 / 39, 27
4. Duplicate this layer and set it to ‘Addition’ at 20%.
5. Duplicate it again and set it to ‘Value’ at 60%. Go Components>Colours>Channel Mixer:
Red: 130, -15, 5
Green: 0, 120, 0
Blue: 0, 50, 60
Uncheck Preserve Luminosity.
6. Duplicate this layer and set it to ‘Normal’ at 50%. Go Components>Colours>Channel Mixer:
Red: 105, -10, -20
Green: 3, 92, -3
Blue: -3, -5, 98
Uncheck Preserve Luminosity.
7. Duplicate this layer and set it to ‘Dodge’ at 10%. Go Image>Merge Visible Layers>Expanded as necessary.
8. Go Colours>Components>Channel Mixer:
Red: 98,0, 3
Green: 0, 100, 0
Blue: 0, 0, 85
Uncheck Preserve Luminosity.
9. Go Colours>Colour Balance:
Midtones: -15, 7, 23
Check Preserve Luminosity.
10. Create a new layer filled with 130a64. Set it to ‘Screen’ at 30%.
11. Create a new layer filled with ff0000. Set it to ‘Overlay’ at 20%.
12. Create a new white layer. Set it to ‘Grain extract’ at 20% and add layer mask. Copy and paste your base into the mask. Invert (Colours>Invert) the mask. Merge your layers again.
13. Go Colours>Components>Channel Mixer:
Red: 120, -30, 10
Green: -10, 110, 10
Blue: -15, -10, 140
Uncheck Preserve Luminosity.
14. Duplicate this layer. Go Colours>Auto>Colour enhance. Set this layer to ‘Colour’ at 30%. Merge your layers.
15. Duplicate your base. Go Colours>Colourize. Use these settings:
Hue: 126
Saturation: 10
Lightness: 10
16. Invert this layer and set it to ‘Subtract’ at 10%.
17. Merge your layers and sharpen (Filters>Enhance>Sharpen).
There you go!