Segmenting pills from the background
If you want to use the thresholding approach, you should use an adaptive thresholding method if there are big lighting variations like in the 3rd example image (dsp question here).

Also, you should experiment with colorspaces, it's easy: the script to decompose the image into different colorspaces should be no more than a few lines long, and a lot of image viewers have that option available. The best one should be easy to determine just visually. If you want to read up on colorspaces, there's anothrt good dsp question here.

Finally, you might want to try a diffrent approach. One idea would be to do a not-perfect segmentation, then an edge detection, and finally using something like Hough transform for circles which also works fine on (incomplete) circle arches. (this idea is ofcourse only applicable to round pills)