What are the blue or green circles on Doppler radar?

I know how to read a radar screen, but have you ever been looking at one and there will be like a blue or green circle that appears for one of the images? The color looks really light, and usually when this happens there is little to no precipitation in the area and it looks like the diameter of the radar. I was just wondering what causes this? Low cloud base or just a non filtered image. This has always confused me. Thanks!