How to Fix Mixed Lighting Color Casts in GIMP

Mixed ambient lighting occurs when different light sources—such as warm tungsten indoor lights and cool daylight from a window—cast conflicting color temperatures across a single photo. Correcting this in GIMP requires isolating the affected areas using duplicate layers, applying targeted color adjustments like Color Temperature and Color Balance, and blending the corrections seamlessly using soft layer masks.

Step 1: Duplicate the Base Layer

Open your image in GIMP. In the Layers panel, right-click your background layer and select Duplicate Layer twice. Rename the first duplicate to "Warm Correction" and the second to "Cool Correction." These separate layers allow you to neutralize each distinct light source independently without destroying the original data.

Step 2: Neutralize the Dominant Cast

Select the "Warm Correction" layer to fix the warmer sections of the photo:

  1. Navigate to Colors > Color Temperature.
  2. Lower the Intended temperature slider to cool the image down until the warm-lit sections appear neutral.
  3. Alternatively, open Colors > Color Balance and shift the Cyan/Red slider toward Cyan and the Yellow/Blue slider toward Blue in the midtones and highlights.
  4. Click OK. The entire image will temporarily look overly blue, which is expected.

Step 3: Isolate the Correction with a Layer Mask

  1. Right-click the "Warm Correction" layer and select Add Layer Mask.
  2. Choose Black (full transparency) and click Add. The correction will disappear.
  3. Select the Paintbrush Tool (P), set the foreground color to white, and choose a soft brush with hardness set to 0.
  4. Lower the brush Opacity to around 30–50%.
  5. Paint over the areas of the photo affected by the warm ambient light to gradually reveal the cooler correction only where needed.

Step 4: Correct the Secondary Light Cast

If other parts of the photo are overly cool from daylight or fluorescent bulbs:

  1. Select the "Cool Correction" layer.
  2. Go to Colors > Color Temperature and increase the Intended temperature to add warmth, or adjust the Color Balance toward Red and Yellow.
  3. Add a Black (full transparency) layer mask to this layer.
  4. Using the soft white brush, paint over the cool-toned regions to blend in the warm correction.

Step 5: Smooth the Color Transitions

Where the two light sources intersect, color boundaries can appear unnatural.

  1. Select the layer mask of either correction layer.
  2. Use the Smudge Tool or a low-opacity black/white brush to soften harsh edges along the light transition zones.
  3. If specific colors still appear overly saturated in the transition, go to Colors > Hue-Chroma, select the problematic color channel, and lower its Chroma slightly.

Step 6: Verify Neutral Tones

To confirm the color casts are neutralized:

  1. Right-click the top layer and select New from Visible.
  2. Open Windows > Dockable Dialogs > Sample Points.
  3. Hold Ctrl and drag from the rulers to place sample points on neutral gray or white objects in both lighting zones.
  4. Check the pixel values in the Sample Points dialog to ensure the Red, Green, and Blue values are closely balanced.