Personally I never look for anything, including dragons, based on the main colour of the character... I also mostly draw a white-red couple... and can't add to any other colour than mixed... 
I prefer medium (in case of dragons I'm only interested in drawings/paintings and digital drawing without textures) or, in case of dragons, the dragon type (based on limbs and bodytype: dragon:4+wings, wyvern:2+wings, drake:4+none, et.c  this sort), or by the level of realism: realistic(everything, anatomy, colours, shading, texturing -all look rather realistic), semi-realistic(one thing is non-realistic: shape, colouring style, line strength/width, whatever), cartoony(pretty obvious I guess), chibi(unproportional, cute and rather simple). Also I'd separate character/species reference sheets and size charts, comics, writing and adopts and add 'other' and 'mixed' folders for dragons fitting no categories or more than one type on the same image.

And, as for medium based, becasue traditional and digital art usually get much more submissions, I'd make separate folders for a few of the main mediums within those (e.g. traditional: penicls, pens, paints, carving/modelling, other).