This was perfect. Thank you so much!

"Dave O" wrote:

Set up your conditional formatting so Condition 1 reads "Formula is"
in the dropdown, then paste this formula (or something like it) in to
formula field, then set up the formatter to paint the cell red.

=OR(C3=1,C3=2,C3=3,C3=4,C3=5,C3=6,C3=7,C3=8,C3=9,C 3=10,C3=11,C3=12,C3=13,C3=14,C3=15)

Some of your codes are alpha characters, so you'll need to enter those
inside double quotes:
=OR(C3="A",C3="f",C3="J"...) etc.