sand is pretty good since it resembles subrtate from nature. I have sand in my JAG tank. Yes they will dig, A LOT. and on color the darker the color the brighter your jack will come out.
sand can be a bit more difficult to manage than gravel. it compresses easily and 'can' develop dead spots if you use too much (more than 1.5-2 inches) and don't keep it stirred up and vacuumed. these dead spots 'can' develop anaerobic bacteria which, if allowed to come in contact with the water column, are a potential and serious disease hazard.
that said, alot of people use sand in their tanks with no difficulty. i mention this to make you aware of the 'potential' problems if sand is not maintained properly. as managuay mentioned, darker is usually better in terms of cichlid color but you may find dark sand difficult to locate.
personally, i maintain 20 large cichlid tanks and use 2 mm natural gravel in all of them. (a) i find it easier to maintain, and (b) those cichlids that are 'diggers' seem to prefer it to sand (except my Geo and Gymnogeophagus which i do keep over sand).