In Essentials, is there a way to create a multiple choice field with a formula that checks to see if a specific selection is made and if not informs the user?
To do that in any version, you would need to create a new text formula field to calculate a warning message text formula. The text result can be colored in red or any color you like.
Quickbase formulas recalculate as soon as the data fields which they rely on are populated or actually event if not yet populated. ie a similar to excel, they are live calculations as data is entered and you tab out of the field or select a multiple choice.
You do have to put the field on the form so that the user is able to see your warning message. I thought that was your question - how to alert the user to a warning condition in their data entry.
I usually put warnings at the top of the form and use a form rule to hide them if they are blank (to close up the extra space). I forget if Essentials supports form rules.