Right-click the layer you would like to create multicolor/multi-line, choose Properties… and go to the Labels tab, and click the “Expression…” button.
For example, to have the New ID on top and the Old ID on the bottom:
[New_ID]&vbnewline&[Old_ID]
To have each a different color:
“
The tricky part was figuring out that you had to put double quotes around the formatted text, and drop the normal double quote down to a single quote around the 255.