Quickmask gets its name from the fact that as you create your selection
area, Photoshop masks that area off, tinting it with a colored mask to show
what has been selected. When you're finished making the selection, you exit
Quickmask mode, and Photoshop automatically converts the mask to a standard
selection so that you can edit the image. Quickmask is a visual, painterly
way to make a selection, and if its handled correctly, it can make unique
selections that are impossible to recreate in any other way.
Before we start creating Quickmask selections, we need to set up the parameters for how Quickmask will work. The following step-by-step will set up the way the painted mask looks as it is applied, along with some other relevant selection parameters.
Since you've already chosen a mask color in step 2 of the above step by step,
you don't need to worry about color anymore while in Quickmask mode. In fact,
Photoshop doesn't even let you work with color in this mode; it converts everything
to grayscale. To see what I mean, try to select a foreground color in the
color palette, it automatically converts to it's grayscale equivalent.
The reason for this is that Photoshop uses grayscale values to control the
relative intensity of the mask you are painting. With black as the active
color, the paint tools paint the mask at 100% intensity. If white is the active
color, the mask is erased, and any shades of gray will paint the mask in relative
degrees of opacity. In summary, black lays down the mask color, white erases
it. And if you make a mask/selection with an 80% gray, then any editing done
through that selection will be applied at 80%. Paint into that selection and
it goes on at 80%, delete that selection, and its deleted to 80%.