When buying professional artist paint, find brands that have single pigments and are highly loaded with colour.
The cheaper the paint the less pigment you are buying. Paint manufacturers that have a high loading of pigment will want you to know. Look for the pigment number on the back of the tube and a light fastness rating. Desirable paint will have few blends of pigments and will only have one or two pigment identification numbers listed. Cheaper paints are full of fillers such as opacifiers barium and surfactants. Next time you are buying paint go for something different and spend a couple of dollars extra on a paint that will treat you well, make painting easier and use paint that goes further!
St. Luke recommends Langridge, Golden Acrylic Colours, Williamsburg, Blockx, Schmincke and Old Holland. Check out the links to see the product range.