When you paint a rental unit you want to think about a few different things. First of all does the color appeal to the masses? You want to use neutral colors. I like beige or gray for walls and white for trim and ceilings. If you have many units it is usually easier to just stick with one or 2 colors and use the same brand and sheens. This will make touch up a breeze. I know some people who use flat paint because it is usually the cheapest. I never use flat paint because it is almost impossible to clean. I use satin or eggshell on everything including the ceilings. This helps for between tenants because I can hire almost anyone to wipe down walls. Not everyone is a good painter. I used to use a local paint company, but since I own properties in many different areas it made it hard to match paint. Also the company I was using would discontinue some of their products, again making it difficult to match. Now I primarily use Sherwin Williams paint because I like the quality and they are everywhere. If you own multiple properties and you are going to use Sherwin Williams you will want to set up an account and negotiate a price. I was charged 3 different price for the same paint before I set up an account. Now I know how much it is going to cost me before I even go in. If you are going to hire someone to paint your units make sure you get multiple quotes. Prices can vary immensely and remember you can always try to negotiate with contractors. Also make sure they know what paint you want to use. I usually just purchase the paint because many times I actually get a better price than they do. Make sure you tell them to caulk all holes and cracks as well. If you have to cover up stains or have a bad smell in an apartment, I would suggest you use a Oil Based primer. Water based primers don't seem to work nearly as well. Make sure to ventilate when using oil based paints and a respirator would be a good idea. When using a oil base primer, you don't need perfect coverage. Put on a thin coat and let dry. Then you can paint it with your final paint.