Titles appear pixellated when in colour

Some of the titles in my video are in purple and some are in white. If I export to h264, the white titles look crisp and clear but the purple ones are a bit pixellated and artefacty. (They all look good when exported at huge quality (Master File) but of course these files are massive.)

What's going on? How can I get the purple titles to look as clear as the white ones?

Specifically what title template are you using?

I tried it with both 'Continuous' and 'Slide Reveal' and got the same effect with both.

That is odd, what if you change the color? Just curious.

I've tried it in pure blue, and the pixellation is still there. Do you want me to try anything else? Happy to do whatever is necessary to sort this out.