The complete list, in ascending order of player level (descending order of speed) is blue, white, yellow, and double yellow. Blue is the fastest, and is recommended for beginners; the "double–yellow dot" ball was introduced in 2000 and is now the competition standard. There is also an "orange dot" ball, for use at high altitudes; this is even slower than the double–yellow.
Some sources describe a green–dot ball, which is the equivalent of the white (slow), and a red, which is between the blue and the green (or white). These appear to be older standards – I'm guessing that the red–dot and green–dot balls were replaced by the white.
Some manufacturers use a different method of grading balls. For example, Dunlop offers the Pro as the equivalent of the double–yellow–dot; the Competition is the equivalent of the single–yellow–dot, and gives 110% as much bounce as the Pro; the white–dot and blue–dot balls are matched by the Progress (120% of Pro bounce) and the Intro (140%).
© Haydn Thompson 2017