The watermark process does not use that function.
The Truecolor thing is used in creating an image not the watermark
Watermarking uses this
Code:
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You can see if your server has mogrify ( Imagemagick ) and use that instead of GD2
PHP: imagecreatetruecolor - Manual
imagecreatruecolor has no setting for bit rate it is only height and width. I do not see anything special that needs to be done for a 24bit png.