You are noticing what many veteran painters deal with as a law of sorts. Whites, flesh colors, and anything desaturated with white is going to be chalkier than your darker paints. This is actual even true for oil paints as well, and with this medium they use a fat over lean rule that, while not intended to battle chalkiness, has the fortunate consequence of making whites less chalky. Basically, this rule says not to dilute your whites, and to put them on thick or impasto. There are two ways to do this with your acrylics. The lighter the color, the less you should dilute it, up to an undiluted pure white. Pure white should be used so sparingly anyway that the thickness of the paint shouldn't really affect the smoothness of the overall finish. Secondarily, as Maxx mentioned, place a larger area of pure white down and then glaze over it to create lighter colors. The key here, though, is how you apply your pure white. You need to scumble it on, which means to put a very small amount on the brush and stretch it very thin with very free, painterly brush strokes. Banshee does this on his famous chaos-type knight bust from Scale75 when he painted the box art. See Figure Painter Magazine if you are really interested in this technique.
If you read nothing of what I have written, read and remember this: Use Vallejo Model Color white paints. My favorite is VMC Ivory. They are the least chalky of all white paints on the market right now, IMO. Others will second this. There are several other slightly off whites from them that work just as well too. Use these whites to lighten other colors and forever rid yourself of chalky light paints.
