Normally when you double click on one of the colour variations on the right you set that colour, but for the 2 colour on the extreme right, when you double click the bar beneath starts to move up or down, instead of selecting them.
Not sure if this is a bug. Click on the squares several times and see they all doing a specific thing.
Double-clicking *any* of those squares does two things: it sets that color as the current color *and* it adjusts *all* colors in the palette of 6 in a specific way, which is unique to each of the 6. The two on the left adjust the Hue, in random increments one direction or the other (top goes clockwise on the color wheel, the bottom goes the other direction). The two in the middle randomly adjust the saturation up (bottom) or down (top). And finally the two on the right randomly adjust Brightness (i.e. "Value"), which is why they're the only ones that adjust the slider to the right of the color wheel. So those each correspond to random variations "up" or "down" along the 3 axis of HSV color. If you want to randomize all values use the Randomize button on the bottom-right.
For all of these randomization buttons the changes can be pretty subtle sometimes so you may not notice in certain cases, and if you press one of them too many times you will get to the minimum or maximum for that particular setting, so that can make it look "broken". But it is intended behavior.