ColorMask is an application for creating masks from images suitable for, primarily, screen printing. Masks can be picked based on color or luminance (lightness).

After launching ColorMask, pick the image you want to use from File -> Select Source..., then click the + icon in the bottom left of the window to create a new mask.

Each mask has two primary types of controls: the Selection Mode and the Halftone Mode. The Selection Mode controls how colors are picked from the image to make the mask, and the Halftone Mode allows gray areas in the mask to be rendered as a sequence of black and white dots.

The starting point for all the Selection Modes is the color that is being selected. Once a color is selected, the different types of Selection Modes control how the program decides whether other colors are similar enough to be part of the mask or not.

It is best to chose a reference color from the image using the color-picker tool. This can be accessed by clicking the color swatch in the control dialog, clicking the magnifying-glass icon, then clicking on the image in the area you want to be part of the mask.

All Selection Modes also have a Falloff control, which controls how sharp the division is between the black and white areas of the mask. A Falloff of 0 will produce very hard edges, while a value of 1 will produce very soft edges.

Perceptual selection mode choses colors based on how closely the human eye perceives them to be the same. This is the optimal mode to use for selecting masks from color images.

Changing the Color slider controls how similar the color must be, regardless of how light or dark it is, before it is part of the mask. Lower numbers mean that colors have to be very similar, while larger numbers mean they can be further apart. The Luminance slider similarly controls how closely the lightness of a color must match.

Luminance selection mode choses colors based solely on the lightness or darkness of the image. This mode is best for black and white images, or images in which the shadows define the ares you want to mask.

The Halftone Modes control if and how gray areas are converted into patterns of black and white which will give the appearance of gradients when printed. If you don't want halftoning, you can select None from the drop-down.

The Dots halftoning mode is the most traditional. It will convert gray areas into a grid of dots, which are closer together the darker the gray. Sliders let you control the base size of the dots, as well as the angle of the grid and the sharpness of the dots. When creating multiple masks to be printed together, it is important to select different angles for each mask.