Brother GTX pro Instruction Manual page 153

(3) If you have selected [Color+White Ink] or [White Ink Only] in the Select Ink operation, you can change the
background color of the preview window and preview the print image. Select [Change...] > [Define Custom Colors],
and enter RGB value.
<TIPS>

When the [White Data] checkbox is marked, the preview window for white ink only will be displayed.
"Example: Preview window for printing an image on a brown (R=128, G=64, B=64) T-shirt"
153
Basic printing procedures