@StephenGW
I can see what you mean, check if below works as a test:
1) Try putting your color change and size change there again.

2) Click on this button: </> as shown below:

3) Now notice you see the HTML view below:

What the problem is now, is that if you click the button </> again right now, it will revert to black, and then if you click it again, it will show some of the code stripped out as well.
If you happen to do that, then repeat Steps 1, 2, and 3 above, and then do the following step instead:
4) Press return at the end of the line to insert a line break, or do any other kind of change of your choice that will grey out the </> button, as shown below. Some changes will not grey it out, so make sure it is greyed out.

5) Check if the issue persists.
I remember that I did encounter this kind of issue before. A long time ago I encountered it with the Outlook Office 365 Send Mail or any other Action that has this similar kind of control. This issue looks similar to that issue I encountered before - maybe it is still there for all those rich text kind of Action editors.
If the above way works, please indicate here, as I did not specifically test it right now for this case.
If the above way does work by the way, I am unfortunately not aware of any other way (i.e. such as a way that doesn't require forcing the HTML view) than just this way I have here.
Just beware if using this way. If relying on this way, then once making it to grey it out, do not switch it back just to edit it with the rich editor for convenience - it will strip the code again and revert it again, and the more code there is, the more code it might strip out. If this way I described here works, I am not aware of another way.
A long time ago, it is possible that with some very specific edits to the HTML, or some other bizarre steps I came up with before, I was able to get it so the editor would not revert back the changes - however, I do not recall the steps (or if they even exist, it is possible I just did it this way here) - or even if such steps existed if they work now anymore. For now please try the above and indicate if it worked at all.
If it works, but you still need solution which must use the rich text editor not to revert back the changes rather than using HTML only, please indicate as well - for now I also have same issue, can reproduce the issue as well, and can only think of above solution for now.