Since the lower layer RGB value change only if the upper layer RGB value are darker(smaller value). You can do a simple trick such as applying 255,255,50 from upper layer to lower layer in darken mode, that will leave Red and Green channel untouch while forcing and green value do not exceed 50.
Instead of immediately showing you how to do a cool layer mode trick that might be rarely use by you, we will explain the true mechanism working behind every layer mode. By fully understanding the fundamental of every layer mode, you will be able to apply layer mode into any problem you might run into when the time come.
Layer mode is about blending the content of two or more layers together to produce intended result. The result of one blending can be both final artistically result or intermediate result that use for further image correction or enchancement.
Layer mode blending the content of layers by calculating the Red, Green and Blue value of every pixels against its lower layer Red, Green and Blue value. Almost all layer mode do calculation on each color channel separately, that is mean upper layer Red value will be only use for calculating against lower layer Red value, and same thing happen to Green and Blue.
While most of the layer mode do calculation on RGB channel, some layer mode do use HSB(Hue, saturation and brightness) for the calculation, e.g. Hue, saturation, color and luminosity mode.
Because the Normal and Dissolve mode are easy to understand, the very first layer mode we explain is Photoshop Layer Darken Mode. Darken mode is easy to understand too, while blending the value of 2 layers, for each RGB channel Darken mode choose the darker value. Note that in RGB 0,0,0 mean black and 255,255,255 mean white.
For example, if one of the upper layer pixel has a value of Red=10, Green=210, Blue=55, and at the same spot the lower layer pixel has a value of Red=150, Green=30, Blue=255. Then the result of blending would be Red=10, Green=30, Blue=55.