Like this, it will actually take quite a while to reach a number that is close enough to the yellow color you've chosen that you can't tell any difference (and mathematically, you will never actually reach an average of 4!)
Preparation, the key to a successful paint job is to take the correct steps when preparing the job. Preparation for this article, has two parts, the first part is wall prep. The walls you are going to paint may have nail holes or imperfections that need to be filled before you paint. The proper way to fill them, if they are smaller is with water putty, by simply forcing a little bit of the putty in the hole and breaking it off flush with the surface. For larger depressions, use a drywall compound or spackle, which may require a second application to fully fill the indentation and make the repair/"patch" flush with the wall.
The reasoning here is simple. Every color in the visible spectrum can be assigned a number based on a luminosity scale (a scale from light to dark) from 0 to 9 where white is 0 and black is 9. Now suppose that the brown you are trying to cover has a score of 8, and the yellow paint you want to apply has a 4.
Store your leftover paint in plastic Rubbermaid beverage jugs or other plastic seal-able containers. Paint left in the can always gets funny after time. Either the can isn't properly sealed of it gets exposed to extreme heat or cold. I like to store my paint inside where the temperature is controlled.
And, its never been more important to know your colors. Why? Because color is back in a big way! Just look at the vast variety of colors in the department stores.
Water-thinned rubber-latex paint is already an old reliable, though it is only about 10 years old. It accounts for a big percentage of all paint sold and is still the most widely available of the easy-to-use finishes. One new type is a combination vinyl-rubber paint that is said to do a better job on interiors than either vinyl or rubber alone because it dries faster, lasts longer and has less sheen.
Once the drywall compound/spackle has had sufficient time to dry, you will need to lightly sand the surface of the repair to smooth out any ridges that may have been caused while "coating" the patch. Once you have the surface sanded smooth, take a wet sponge or cloth and, by lightly washing/rubbing, blend the outside edge of the drywall compound/spackle back into the wall. For a smaller repair, you can wash away the excess drywall compound, leaving only the compound/spackle only in the area where the indent was filled. For a larger hole, after filling, sanding and using a wet sponge to blend the outside edge of the compound back into the wall, it may be necessary to texture the area in order to give the smooth patch the appearance of the existing wall.