What is the meaning of the available color lists in Bricklink searches?
Lots for Sale - this contains colors in which current part is being listed for sale.

On Wanted list - this shows which colors Bricklink members have added to their "wanted list". It doesn't mean parts in this color have existed. Most likely this being used to add non-existing color to list of wanted items and mark for notification. In such case if wanted item ever to appear for sale (and seller pushed this notification) person with desired item would be notified.

Price Guide info - here you can see sale/price history for each color for the past 6 months as well as items currently listed for sale.

Known Colors - here you can find colors which appear in released sets. Bricks in this list are tied to set inventories produced by community of Bricklink. If there ever be an error with inventory or it has alternative selection of bricks, this is being addressed by fixing or adding an alternative inventory for the same set. Every Bricklink member is welcome to make these fixes.

There could be several reasons why there are more colors for sale than on a list of known colors:


An error is an obvious thing. It is quite easy to make a mistake while listing individual part for sale if it has been produced in similar colors, or has variety in shape (with/without groove). Some shades of Blue and Pink are hard to distinguish. Shape differences aren't always so easy to notice too.
Some parts you see for sale may have been prototypes, test bricks or
produced for specific project (like life-size LEGO Bugatti has a ton of non-existing parts) which may have made their way somehow from the factory. This is not something you could request for as LEGO is trying to not let these items into the market.


One more thing to note. If you find a part in odd color and it isn't listed as HTF (Hard To Find) or Special Color, this is most likely a mistake while listing. Odd parts have quite a high value (depends on availability). Although it doesn't hurt to contact the seller to confirm the case before ordering.