Making a custom field drop-down selection stand out if selected

[#1]

We have a custom form field called "suppliers" and it contains hundreds of choices. Certain suppliers have special requirements that the people working on these projects need to know about. It would be so nice if we had the ability to flag certain suppliers within the custom form so that, when selected, that field stands out as different from other selections - such as turning red, having a flag, etc. to alert the workers to view another document.