I never did remember what the different colours on squash balls meant, but found a handy guide on Wikipedia…

Different balls are provided for varying temperature and atmospheric conditions and standards of play: more experienced players use slow balls that have less bounce than those used by less experienced players (slower balls tend to ‘die’ in court corners, rather than ‘standing up’ to allow easier shots).

Colour Speed Bounce
Orange Super Slow Super Low
Double Yellow Slow Very Low
Yellow Slow Low
Green or White Medium/Slow Average
Red Medium High
Blue Fast Very High

Source: http://en.wikipedia.org/wiki/Squash_(sport)