High-quality wood cabinets call for matching molding. Don't paint it a different color if the cabinets are solid oak with a professional stain, for example. It's typically not a good idea to make the molding stand out in this situation. You want the cabinets to look seamless all the way to the ceiling. High-quality painted wood cabinets may require matching crown molding as well.
Using two colors of cabinets calls for a different technique. If you use stark white cabinets in the top half of the room and dark cherry cabinets as base units, you'll want to tie the two colors together. By installing crown molding stained to match the cherry base cabinets at the ceiling level, you will create a "wow" factor.
Use colored pencils to draw different techniques. You can, for example, use white upper cabinets and hunter green painted base cabinets. Instead of painting the ceiling crown molding hunter green, however, you might want to paint it pale green. Use this same pale green on countertops.
Create techniques involving backsplash materials. If you install cream-colored upper cabinets and black lower cabinets, you can use copper colored tin tiles in the backsplash area. You can paint the upper crown molding at the ceiling level with copper metal paint to match the backsplash.
Two wooden stains can call for two shades of crown molding. You can alternate two maple cabinets with two cherry cabinets in upper and lower sections of the kitchen. At the ceiling level, you can run crown molding using cherry stain and a section of flat wood trim in maple finish just under it. Consult an expert carpenter at how to do this, so your end result is very professional-looking.