1. Big Room = Light and Neutral colors; Small Room = Saturated colors
Contrary to popular belief, small rooms handle bold colors better than big rooms. I am a big fan of choosing deep saturated colors for bathrooms, pantries, closets; I also love using bold wallpapers in those spaces as well. I recommend that you try painting or papering the ceiling to match the walls in those smaller spaces. It can truly open up a space and make it feel larger.
Big Rooms, in my opinion, should feel big and spacious, so keep it neutral with your paint color selection. I always recommend that clients use furniture and accessories for pops of color in those spaces.
2. Orientation of a room
I am a firm believer in choosing the warmth or coolness of a particular color based on the orientation of the space. If the windows in a space are facing south or west, I highly recommend going with a cooler version of the color you are thinking of using. On the contrary, if you are facing North or east, try a warmer version of that color. The natural light coming in the windows will even out the color temperature in the paint.
3. Trim Colors:
I typically recommend that clients pick one trim color for their entire home. This is for two reasons: One, it keeps the color palate throughout your home consistent, i.e. every room color you choose needs to work with that trim color. This allows you to plan how the colors blend together overall in your home. Two, when you have to do touch-ups down the road because you slammed your vacuum one to many times at a particular corner(s), you don't need to go looking for a particular can of paint; you can simply have one can of paint waiting for all of your touch up needs!
4. Color selection in a room: Samples on the wall
I prefer to guide clients though a saturation selection of colors if they are unsure of how a color will look on their walls. For example, a client really likes a particular shade of brown, and I am concerned it is too dark for the space. I will have the paint shop mix that color, one shade darker and one shade lighter (sometimes even 2 shades lighter). Then we put a 3' by 3' square on all of the walls in that room. This allows the eye to understand how the various shades look in a space. Then take a look at the samples over the next 24 hours, this allows one to select a color based on how it looks through out the day and night. It may surprise you to see how much a color can change from natural daylight to incandescent in the evening and night.
5. At least 2 Coats!!!!
Now that you have selected those colors and trim and finish, it is time to get to work. Professional painters will always paint the trim first, then run the color around the room adjacent to the trim, then roll on the color. This process needs to be repeated at least one time, i.e. two coats.
I am not a fan of paints that are advertised as a one coat paint, this means it is a paint and primer in one can. I have never had success with these products, nor have any of my professional painters, with whom I work.