When trying to decide on a wall color, don't even try to decide on a specific shade at the start. Instead, choose a primary color you think would look good in the room. This provides you with a starting point from which to choose individual shades, rather than forcing you to think of a specific shade right from the get-go.
Every hardware store with a paint section will have samples of every shade it carries on color cards or swatches, including shades of the primary color you chose in Section 1. Grab as many of these swatches as you think will look good on the wall you plan to paint. They don't cost anything and will allow you to compare them to the colors that already exist in the room to see how the two colors will look together.
Take the paint swatches home from the hardware store and start putting them against the wall to see which one looks best. Make sure you also put the colors against other major pieces in the room -- furniture, carpeting, tile and so forth -- to see how the colors mesh together. It may be that "steel gray" looks fine on the wall by itself, but it may not look so good when placed against your ruby-red sofa cushions.
Not all paint brands are made equal, nor do they all carry the same shades and hues. If you can't find the exact right paint you want for your walls, check other stores and other paint brands to see if they have something that will serve your needs.