vallejo game colors are great imo. they are slightly cheaper per can, but the bans are also slightly bigger, and most importantly, they do not dry out ever.
i have only recently started trying non-citadel colors, and so far vallejo game colors have been kind to me. i do not think that there are any replacement for badab black and devlan mud, but other than that they should pretty much cover anything citadel has.
here is a chart for converting citadel color names to the coresponding names of some other brands. its not a perfect chart, bu its decent.
vallejo (and plenty of other colors) can be bought at http://www.maelstromgames.co.uk/
great site, with free shipping and good prices.