I've been trying to find an easy way to turn off colours in Safari, but I can't find one, either. My dislike for that browser has just increased another notch. It is so far from being user-friendly that I won't use it.
Another poster had the same question, but he was using Firefox, as I do, which makes it child's play to turn off the gray fonts on this board. I do each time I come here.
This link is to that thread that also has a clickable thumbnail pic attached to it.
If no one has an easy answer to the Safari dilemma — and I certainly could be missing something obvious — and you'd like to try downloading Firefox, let me know here or by a PM, and I'll talk you through the process if it's confusing.