You can make subtle adjustments to the color saturation of pixels on a texture image using the Sponge tool. Sponge works by increasing or decreasing the color saturation on pixels in the areas you stroke.

Sponge is based on a Hue, Saturation, and Value color model (HSV), where the saturation value for pixels in a paint layer are modified in the areas you stroke.

To adjust color saturation

1. In the Paint Layers window, select the paint layer you want to edit.
2. In the Paint Tools tray, select the Sponge tool.
   
3. In the Sponge tool properties, set the Amount to increase or decrease the saturation for pixels.

   A positive value decreases a pixel’s color saturation, while a negative value increases it.

4. Stroke across the painted texture in the areas you want to affect.

   Color saturation for the pixels is modified in the areas that you stroke. Hue and Value are not affected.

   You can temporarily reverse the tool effect by pressing the Ctrl key as you stroke. When you press Ctrl, if the Amount property is currently positive, it is temporarily set to the inverse negative value. Stroking repeatedly in the same area causes more buildup of the saturation or desaturation effect.