The doors are most likely painted with lacquer, which over time has yellowed, this problem is even greater if someone in the house smokes. This is something you can do in your garage.
Wash the doors with a degreaser and dry right away, scuff with ScotchBrite (Red) veryfine until there is no shine on the doors. You have a choice brush or spray.
Go to a paint supply store (ICI-Dulux, Kelly-Moore, Sherman-Williams, etc) NOT a home center or hardware store. To brush: Purchase a good 2 1/2" sash brush $20-30, A Qt. of flotrol, a gallon of waterbased Sealer/Primer (BINs), and a quality 100% acrylic trim paint or a modified acrylic (do not buy wall paint), lay the doors flat face down on small blocks so the edges don't touch the bench, Mix flotrol into sealer and paint in light coats, repeat if needed, Flip doors and paint face. Let dry lightly scuff with 220 paper or scotchbrite if needed. Paint in same manner.
If you want to spray them rent an HVLP Gun and turbine (check with paint store or rental yard) Omit the flotrol and follow above. Remember overspray, cover it if you don't want it white. DH