Q:

circle with dfferent colours representing different criteria

I want to draw circles on maps with different colours, this gives the signal strength of the tower, suppose circle is of 50 m then first 20m should be given a color1 and next 20 m color2 and remaining color3. 
Is this possible in Geoserver SLD or openlayers? How?

A:

Look at that! Someone has already asked for drawing circles and it has already been answered.
Regarding colors, you should use OpenLayers.Style(); and have a look on OL doc and eventually on the wiki page.

