Can I paint dark cabinets grey or should they be sanded first

First clean the cabinets especially if they are kitchen cabinets. Scrub them with tsp to get any grease and cooking residue off. You don't want to sand and grind that into the paint because then you will have paint adhesion problems. You should always prime with a heavy weight primer in white or light gray. It helps level the surface. After you prime give a quick sand by hand to remove anything that might have gotten in the paint and to level the surface so it is all smooth. Paint with at least 2 coats of whatever color you are using with a light scuff between coats.

http://www.diynetwork.com/how-to/skills-and-know-how/carpentry-and-woodworking/painting-kitchen-cabinets

The prep is key to a good looking and long lasting transformation. If you care about the way they will look after then you need to take the doors and drawers out/off and remove hinges and knobs then scrub them down with a degreaser, rinse, wipe dry and allow to dry completely. Then putty any dings and holes you don't want and sand until smooth with #200 grit sandpaper. You don't need to remove the top coat, just sand enough so the primer can get a grip on the surface. Wipe off the dust well with damp rags before priming. I'd prime and if there's a big difference in color between cabinets and new color have the primer tinted to the finish color.

Sounds tedious but it's worth it. My little 23 knob kitchen took 3 hours to prepare to prime. Be sure you have a space cleared out so you can move efficiently.

Washing, sanding and priming always produce the best surface for paint. Have your primer tinted a shade or two lighter than your finish color, it will help reduce the number of coats of paint needed.

I would prime first

Depending on the paint you want to use, sand or don't sand. I'd definitely clean the cabinets. I use Windex. It's fast and cuts through grease. I'd use a chalk paint. If you don't want brush strokes to show, make sure to use a roller. I use chalk paint a lot and I love it. A trick I learned whole using it, is to wipe over it to remove any leftover dust from sanding it. This also smooths out the paint. You don't have to sand it, but if you want an aged look, which I do, then a light sanding will get that. I use Rust-O-leum. They also make a clear matte for sealing. Make sure you use even coats of the clear and watch out for any drips because it I'll yellow if it drips or if the clear coat isn't applied smooth.

Sorry, the other reason I use chalk paint is because you don't need a primer!