Subtract darkens the base layer by subtracting the colors of the blending layer from it. The result can be dramatically darker, with strong color shifts when the two layers contain different colors.
The Math
For each RGB channel, Subtract takes the numerical value of the blending layer and subtracts it from the corresponding value of the base layer. If the result would fall below zero, it is simply set to zero.
Mathier Math
The basic operation is:
Result = Base − Blend
For example, if the base channel has a value of 0.7 and the blend channel has a value of 0.4:
0.7 − 0.4 = 0.3
The resulting value is 0.3, making that channel darker.
If the blend value is greater than the base value, the result would be negative. Since RGB values cannot go below zero, the result is clamped to 0.
Core Rules of Subtract
Black disappears: Pure black has a value of zero, so subtracting it changes nothing. Painting with black on a Subtract layer leaves the base layer unchanged.
White subtracts everything: Pure white has the maximum possible value, so subtracting it drives every channel to zero, resulting in black.
Negative values become black: Any channel that would produce a negative result is clamped to zero.
Color can shift: Because each RGB channel is subtracted independently, colored layers can produce strong and sometimes unexpected shifts in hue as well as darkness.
When to Use It
Stylized shadows: Subtract can create much harsher, more dramatic darkening than Multiply, making it useful for graphic or experimental shading.
Color effects: Colored Subtract layers can produce unusual color shifts, making the mode useful for sci-fi effects, dramatic lighting, and other experimental color treatments.