What's the big deal about grays and whites you ask? Everything. Atmosphere between you and and an object you are observing makes the chroma of that object somewhat gray or in other words, less intense. For representational painters understanding how to gray a color is essential.
White objects are effected by the temperature of the light on them. Warm light of pure sunshine is warm where light reflected from the blue of the sky filtered through clouds are cool. Reflected light off a surface also effects the temperature of white. Short explanation but there is a lot more to it.
Now Gamblin Colors has created grays and whites that help with the issue of changing intensity and temperature of your colors. For a more complete explanation of their colors click here.
A big thank you to Scott Gellathly for a well written article and for including me as one of the many who use Gamblin colors.