Hello SP#7,
Thank you for coming to the forum and I will be happy to assist in the best way I can.
For me, the whole point of the message is to come up when they click on the cell. There is no way to make the cells have a different color using conditional formatting that I know of. You will have to color them manually. But i think that is an overdo of what you have already achieved with the text input message.
You can go further and do data validation on the cells to make them accept only a type of input. https://support.office.com/article/29fecbcc-d1b9-42c1-9d76-eff3ce5f7249
If there is anything else you need let me know.
If none, have a wonderful rest of your day.
Kind Regards Fuad