Again, that would depend on what field you are changing the value of.
If you want to alter the value returned by the select field then you should use acf/format_value with a priority > 10. If you change the value of the select field before ACF formats it then it will return NULL if that value is not one of the selected options.
On the other hand, if you want to select field to return a URL then just use the UEL as the value when creating the options of the select field
{URL OF IMAGE 1} : Red
{URL OF IMAGE 2} : White
