Paint > Setting Brush Attributes and Modes > Brush Attributes > 
Brush Opacity
 
 
 

The brush opacity affects the transparency of the brush. A value of 100% applies a fully opaque colour. Use a lower value to apply a more transparent colour. Enter a value in the Opacity field.

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(a) Current Colour field

NoteYou can only use the Front, Back, Result, and Saved attribute modes with the Opacity attribute.