This depends on the colour of the denim. We all know black has a slimming effect but I wouldn't say there is one "best colour" per say.
Add your own personality to the mix. A neutral grey denim skirt with a grey legging and monotone footwear (the key is contrasting textures) can look fantastically chic, but it's also super fun to rock a bright neon legging or crazy print! Or take a page from les parisiennes and wear layer upon layer of differing (sheer, opaque, knit) blacks. Let your comfort level guide you.