What you are most likely seeing is a build-up of hard water scale. Look in your tea kettle and see if you find the same substance there.  Hard water scale is the result of water precipitating the natural calcium or magnesium compounds from your water.  Water that is hard is not a health threat, it is more an esthetic issue.  Some people prefer the taste of hard water to softer water.  The colors you may be seeing are the iron (reddish) or copper (greenish) also precipitating out of your water.  If you have your water tested for hardness you can interpret the results as follows:

0 ppm Very Soft
50 ppm Soft
125 ppm Hard
200 ppm White buildup will occur
250 ppm Very Hard