How to concatenate the R,G,B channels histograms to single image histogram?

I have generated superpixels and need to calculate the histogram for the color image.Computed three color channels G=R=B=48*256(48 superpixels and 1-255 range(Uint8 image)). Now I want to concatenate the three channels into one.

For example, pdist(SP1,Sp2,'emd'); How to calculate the distance between two superpixels?

[KSSV]

You can concatenate using

iwant = [R G B] ;