i pretty much made up my mind for vallejo colors, but i'd be curious to know if there also other decent alternatives.
Vallejo's Game Color line just about replicates the standard "Citadel" paints line. They don't have the same types of paints in Citadel's new range (base, shade, layer, technical...), but rather have a fairly consistent paint quality throughout (red tends to be slightly thicker, yellows slightly thinner, in my experience).
Vallejo also has a "model color" line that is geared towards historics. Where Game Color are bold, Model Color are more subdued and realistic. Model color has colors that aren't available in the Game Color line, so they're good on that front as well.
Another brand that is good is the Privateer Press Formula P3. The colors here are somewhere between the boldness of Game Color and the muted Model Color. The colors are also different from Game or Model color, offering up an even wider selection.
GW/Citadel/Game color is for high fantasy and the colors reflect that.
Model Color is for historical games and the colors reflect that.
Formula P3 is for low fantasy/steampunk and the colors reflect that.
My advice is to buy the colors you want. Some people prefer to buy starter sets and work from there.