Sorry, but I must be a real idiot because try as I might I cannot find a combination to complete this puzzle, which means I cannot continue on until I do. Anyone have a solution? The only walkthrough I have located did not supply info because, and I quote "I had no difficulty here."
Guess that leaves me hanging unless someone can help.

If I recall this correctly, this is how it goes. There are six stones in the central pool - two pairs each of three colours. Take a pair and put one of each in a pool of secondary colours. Now go to each pool of primary colours and depress one of the stones (or levers, or however it works) of a primary colour that will make up the colour of the stone in the secondary pools. Go to the other pool of primary colour and depress the other stone.

For example, take the two purple stones from the central pool. Put one purple stone in each of the pools of secondary colour. Now, since blue + red = purple, depress the red stone in one of the pools of primary colour and depress the blue stone in the other pool of primary colour. Voila! Riddle solved.