Brother GTX600NB Instruction Manual page 87

Replace RGB=255 in bitmap image data with RGB=254
RGB=255 is identified as a transparent color. To a portion where RGB=255 is contained, no ink will be ejected.
In order for the white ink to be printed, select an area for RGB=255, and replace it with RGB=254.
To make such replacement, use the method mentioned below.
(1) In the menu bar, select [Effects] > [Adjust] > [Replace Colors].
(2) From the [Original] pull-down menu, set RGB=255 in [Replace Colors].
(3) In the same manner, set [New color] to RGB=254.
(4) Check the [Single Color] check box, and click on the [OK] button.
<TIPS>
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For the vector data, change the fill color to RGB=254.
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Creating print data