Well, any color segmentation algorithm might have problems if the light intensity is changing. For best results you should put in some known color standards, like the X-rite Color Checker chart. If you use that to calibrate your colors, then you'll have less trouble.
That said, the LAB and HSV color spaces separate the color gamut into three dimensions, one of which is an independent intensity axis. So if your intensity changes, you might just look at the S and Hue channels and ignore the Value (Lightness) channel.
Or you can try the Color Thresholder on the Apps tab of the tool ribbon.
