Create a custom mask

Use the Raster calculator tool to create a custom mask.

The expression below will output a new layer, with pixels defined as True assigned a 1 and all other pixels assigned a 0. The Apply a mask tool can use this new layer to mask out pixels where the red band and green band values are both greater than 0.2.

To help you understand what values to use in your expression, you can Calculate histograms in your layer settings.