Forum Discussion
Nobody here will know why Microsoft is offering such as limited set of colors for Text Highlight. You can use text shading instead, if you want (but it won't be exactly the same, obviously).
I recommend that you post feedback for Microsoft and suggest that they add a wider range of colors. For instructions, see https://support.microsoft.com/en-us/office/how-do-i-give-feedback-on-microsoft-office-2b102d44-b43f-4dd2-9ff4-23cf144cfb11.