Right now that is described as in the new guidelines:
The description of the old "guideline" is:"Use the same color range for the whole tileset"
Which is kinda long, and a bit outdated IMO. As for the new guidelines I do not want to have such big text in it (It will make it look messy). If possible I just want to have a few sentences, if possible with examples. Those sentences must make it clear how a tileset should look. A bigger explanation can be given at the Tileset Development page ofcourse.* Regarding color choice: Keep your color palette narrow. Don't use more shades per color than necessary. 3 shades of one color are usually enough for small objects. Even very large objects shouldn't use more than 5 or 7 shades. But how many shades you use is in the end left up to the artists discretion. Keep in mind that dithering is usually used for structure effects in TMW and rarely for blending one color into another. When a structure effect is not needed you shoud better add an additional shade instead of applying too much dithering just for color blending.
* Try to use the same color palette for the whole tileset. The selection of color is important for the feeling of the tileset. Cold colors with a low saturation are good for a sad and cold feeling for example while warm and saturated colors create a warm and cheerful atmosphere (but please don't overdo the saturation). Whatever atmosphere you choose, it should be consistent through the whole tileset. When you create new tiles for existing tilesets you should try to use only colors that are already used in the tileset. Otherwise your new tiles will stand out and look weird when next to the old tiles.
As for the restricting part, how strict should it be? If it is not strict enough the game will not have a consistant look, if it too strict the artist will feel it is to limited for him/her.
Please post your thoughts about this topic.