Colored background for parts of the text has been a basic feature that has been asked for years but I think it’s still not available. The mark tag is not for background color, it’s for highlighting text so it appears above the text instead of below. In your case, I think your color code is wrong because you set alpha to zero so it’s not visible. Try with the one in the documentation “#ffff00aa”.
If you’re not satisfied with the highlighting and you require colored background, there is a hack to make it work in this thread : Text background color on some letters
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