Change Cell color based on greater than, less than formula

Please forgive me as I'm very new to Excel and learning as I go. I'm trying to track fleet vehicle mileage and service using vehicle #, last service, current mileage, due mileage. I would like to have the vehicle # cell change color based on how close a vehicle is to needing an oil change. I've tried setting up a Conditional formatting rule just to due two colors but no cells actually changed color. What I'm trying to do is a bit more complicated and I'm not sure if it is possible or how to do it. I'd like to do three colors, Red if current mileage is greater than or equal to due mileage, green if current mileage is less than due mileage minus 1000, and yellow if current mileage is between due mileage minus 1000 and due mileage minus 500.

is this possible? can someone help me?
Vehicles.xlsx
Thanks so much for you help and sorry if I'm wasting time.

try it now...

for gensets I set the yellow color +100...

just change any value and it will color the 3rd column...

Thank you so much. Works great!