Searching also, I found the following share. It meets the need until 'hopefully' MS builds into OneNote.
How to colorize OneNote textbox (table) content
There is away, a hack.
For an existing paragraph, copy the text, and cut it from the paragraph.
Then, insert a 1×1 table into the paragraph. Next, paste the text into the single cell table.
Click on the table and select the Layout tab in the OneNote ribbon.
In the Format section of the Layout ribbon, select the drop-down arrow for shading. Now you can pick the color that you want for the paragraph.
For a new paragraph,
insert the 1×1 table first and just start typing into the table cell.
By default, OneNote will add a new table row for each hard return of the Enter key.
A way around that is to press Shift-Enter twice at the end of the first line and arrow back up the first newline. As long as you leave one empty line at the bottom, pressing the Enter key will keep the text in a single row.
Now you have colored paragraphs and the color matches the size of the text. And with the default borders for a table, you get a simple box drawn around the text.