Brother GTX600NB Instruction Manual page 111

Transparent color
With this option, specify a color which is treated as a transparent color without using any color during the printing
process. If you want to specify other colors excluding the RGB=255 as "Transparent color", perform the following
procedures to make your setting.
<TIPS>

The RGB=255 is treated as a "Transparent color" (non-printing color).

You would be better off making [Use background black color (for black T-shirts only)] OFF to produce better colors.
(1) Click the [Transparent Color:] checkbox.
Then, the pull-down menu for color selection becomes active.
(2) In the pull-down menu, select a color which serves as a background color of T-shirt, or click on the [Custom...] to
specify a RGB value.
<TIPS>

This custom color you have specified is valid only for one color which is specified immediately before. This color cannot be
saved in the list.
Tolerance
Specify the gradation range (RGB values) to smoothly connect the zone between "Unprinted color" and "Printed color"
to be gradually changed in tone.
<TIPS>

If this value is increased, the gradation range in which the mask white is generated from a state in which no mask white exists will
increase.

The gradation's image quality depends on image data. For this reason, it is advisable to perform a test printing beforehand.
4-34
Creating print data