EMD (and histogram comparison in general) is an interesting approach because it seems to operate on a certain superficial level of "image similiarity". Both, color-based and texture-based EMD methods could end up finding images which have very similiar histograms, but really have fundamentally different meanings (such as p14, fig 7, picture a3). I guess its up to the end user to decide what kind of similarity he is really looking for, superficial appearance (such as images with a similar color-signature), or something deeper, more meaning-based (such as "all images that have the same kind of people/objects performing the same kind of actions under the same kind of environment").