SOLVED
@siankerr if you shared the sheet it would be easier but what you want is Conditional Formatting
I selected NEW and then Use a formula as shown in the second picture to say if the sum = the value they enter then turn the cell green.
I also added a if not correct AND not blank then red
Lastly, you could also choose to have a checkmark added using the built in formatting but then you need to tell it what value is correct in each case instead of using a formula:
