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Currently if a formula field will return only a date or a number, Airtable will have formatting options for that formula field. However, if the formula will return a string or might return a string, or might return anything other than a date/number, there are no formatting options.
Sometimes I write a formula so that it will usually output a date or number, but will return a string if an error condition is met. For example, a formula might calculate the number of days between two dates (a number) but output a string if the end date is before the begin date.
This product suggestion has multiple parts to deal with these situations:
Once formulas can report errors as errors, many formulas that sometimes return strings could then be considered to always return a number or a date or an error or blank, and thus could have formatting options.