Color segmentation and region growing

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Hello, I would like some help regarding the color segmentation. I'm trying to identify some types of blood cells and some of them are distinguished by color. I was already excel in some images, the problem is that as the pictures do not have the same quality, are not equal, the colors change. I wanted to know if has as to do something like set a range of colors... Or if someone can explain to me a little better based segmentation regions growing using simplecv ... Thanks!