Another thought: Actually looking at your second sample image another option rather than changing the watermark color might be if it's too dark then just write it first 2 pixels larger in a lighter color and then write the dark version on top (so it'd be outlined).
WoW! That's some selection of possible solutions from you Fletch. Thank you, it is certainly giving me some ideas to try out.
The idea of an outline by printing the text twice in different colours certainly has plenty of merit. Potentially it would eliminate the need to sample the background at all. If there's a dark outer part to the text and a lighter inner then one or other should contrast with the background sufficiently.
I may have to find two suitable TTF sources or find one and make a copy with slightly narrower font strokes so one sits inside the other. That shouldn't be a big problem.