I apologize, I have led you astray. I should have thought about your OP more carefully before I answered.
The formatting you want to accomplish needs to be done as a label expression. You can do all sorts of neat things with it, but idea would go like this in Arcade:
"<CLR red="255">" + $feature.OBJECTID + "</CLR>"
This bit of code would make the field (objectid) I pulled from my data set red. You can see the full list of text formatting tags and how they work here: https://pro.arcgis.com/en/pro-app/help/mapping/text/text-formatting-tags.htm
You can also use it to build up conditions so that IF a value is greater that x, do this IF NOT do that.
All that is covered here: https://pro.arcgis.com/en/pro-app/help/mapping/text/specify-text-for-labels.htm
Again I'm sorry for sending you down the wrong path initially, hopefully this will get you back on the right one.