Is the mean of visible color in CIELab color space can be non visible color? I am converting color from RGB to CIELab to train a GMM. When I get back the mean of clusters of the GMM I have sometimes negatives values in RGB space. I was wondering if the mean of only visible color can be non visible color ? Therefore I was also wondering if the visual color of CIELab forms a convex shape. 

EDIT: 
I'm using Expectation-Minimization of OpenCV to train the Gaussian Mixture Model.
You are implicitly using sRGB, a small color space. You might like to try using scRGB, a similar color space that allows such values. 

The average of visible colors should also be visible, however the calculation should be performed in a perceptual color space for the result to be more meaningful; L*a*b* is better than sRGB.

Another question to ponder is whether the centers make sense in L*a*b*. Perhaps you have a bug?