IF you use color chips (same tones with 5 shades) to pick your contrasting colors my experience is that it works best to pick 2 that are separated by 1 color (i.e. 1st and 3rd, or 2nd and 4th shade). This will give you some contrast. Multi color applications can be more interesting but I highly recommend getting an artists pad of sketching paper and practicing on the paper to get a effect you like before doing it.
Start out with a base coat of semigloss or satin. I usually use a cream colored off white. Mix a medium deep color up in the shade you wish or mix colors you have already or get some new paint in semigloss or satin interior. If the wall looks great to start with proceed. For a nice subtle wash mix you accent color 1/2 qt. with 1/2 qt. of glaze (water based) Behr, Zinnzer or? and then mix it together. Pour into a paint tray and apply to the walls with a 7" pad painter. Put it on thin (you can add water if you need it to have less color) and by spreading it out there will be darker and lighter areas depending on how much color is left on the walls. The paint will be see through leaving a laquered look.
Get one of the double roller (I like Paint Magic) set ups. These come with vcr or cd-rom demo tapes. When using this system you don't have to base coat as you are blending two colors without glaze so they will cover. They have a number of special roller heads from marble (blocky shapes), rag, sponge, thick nap (woolie) etc. I like using mine with glaze mixed paint but in this case the wall has to be clean (you have to basecoat if it is flawed) as defects show through. It is very fast to use and gets fairly even results. If you don't like a spot, re roll it. I have cut 3/4 nap roller heads with a hacksaw to fit and also the same to a full 9" sponge roller head. The more you roll over it the more the colors blend so the effect will can be strong or subtle depending on contrast in colors and amount of blending.
Using a sponge is the most widely known interesting technique. You can use a cellulose (household) sponge but the nicest I think is the widely available natural sea sponge. Start by laying a basecoat of satin or semigloss down. Mix your glaze by mixing 1/2 glaze with 1/2 paint and start sponging it on. I prefer it densely sponged. Another way to do it is by spreading the glaze mix on with a pad painter. Spread it thin and quickly, working a 3' square area and then use the sponge to remove the glaze exposing the base coat. The sponge will load up with glaze making it necessary to dip in a bucket of water and wring out to start again. Both methods work well. Experiment on paper. Choosing colors is important once again. I prefer the subtle differences in shade from the same tone. Feather dusters and plastic bag stamping can be interesting. Using a rolled up rag and twirling it down the wall makes a nice pattern.
All these hand tecniques to me are time consuming and require more finesse to work the glaze quickly to keep a "wet" edge going, than the double roller.
What ever you decide try playing with your ideas on artists sketch paper first. When you are satisfied, the painting part will go quickly and you will be happy with the results. It is fun to experiment with color and effects.
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