By sending a certain sysex message to the Mastermind GT, you can change the text of any display, and the color of the button displays.
The format is as follows:
F0 00 01 5B <id> 1D <state> <page> <button> <color> <text> F7
Where:
<id> is the sysex ID of the unit, which can be found in the MIDI menu on the GT itself. (NOTE: this number is displayed in decimal on the GT, you will need to convert it to hexadecimal. For the easy way out, set the Sysex ID on the GT to 0. Then you can just specify it as 0 no matter what the numbering system is)
<state> is 0 for button off, 1 for button on, 15 for button disabled. If this is a cycle button, values 0-3 correspond to the four steps.
<page> is page number (0 = page 1, 1 = page 2, etc.)
<button> is button number (0 = upper left, 1 = button to the right of button 0, etc.)
<color> is the color number (0 ... 15) (if you're enough of a nerd enough to speak binary, bit 0 is red, bit 1 is green, bit 2 is blue, and bit 3 is half brightness)
<text> is one or more bytes of text to display in the button's display. This can be left out and the rest of the command will still work. If there are no text bytes, the button's text will be unchanged.