GIMP is a photo editor that allows you to change the appearance of your picture using myriad tools and filters. One of the manipulation tools it offers is the colour balance tool. This tool shows the balance between each primary colour and its complementary colour and allows you to adjust this balance to create colour effects or to remove colour casts. It is especially useful for removing the blue cast daylight creates on a tungsten-balanced photograph.
Open your picture in GIMP.
Create a duplicate of the background layer and name it "Daylight."
From the "Colors" menu, select "Color Balance."
Select a range to adjust. GIMP offers three options: shadows, midtones and highlights. It is usually best to start by adjusting the midtones.
Drag the "Yellow-Blue" slider to the left. This will offset the blue cast by adding more yellow. The picture is properly adjusted when it has neither a blue nor a yellow cast, but looks neutral. A good starting point is between -20 and -30.
Repeat this process with the shadows and highlights, if desired.
Correct the luminence before the chromenance. Once your luma is correct, the colours will be fully saturated and easier to adjust. Also, make sure both "Preview" check boxes are selected so you can see your changes as you make them.