I can't be sure about this, but I think it may be your laptop screen or whatever device you are using that is beginning to fail, color-wise. Even more likely is a cable problem between your monitor and your device (if there is a cable).
If it's a computer/monitor combination, try jiggling the VGA cable (it often has blue connectors on both ends) You might see the colors flash to something that looks better briefly, meaning the cable has established better contact momentarily. Sometimes it's even just loose, and plugging it in more securely at both ends will solve the problem.
If you're not in the US, which your spelling of "colour" makes me suspect, maybe you don't have the same kinds of cables I am used to, and maybe this isn't helpful at all.
At any rate, I don't see the colors you describe, so I don't think you should be seeing white letters on yellow in most places on Wikitree, unless you or someone else has changed the computer color scheme on whatever device you are using. With luck, maybe you just have a loose cable and tightening it will solve the problem.