I cannot seem to find any details on what these colours are supposed to indicate or be related to.
As far as I can tell they don't appear to correlate to anything, and multiple instances of the same MBean interface have different colours assigned to the same attributes.
Like Alexandre says, they are used in the charts. If you add an attribute to a chart, you can right-click in the chart legend and choose to change the color for that attribute, this will also update the color on the MBean Browser page. If no specific color is set, we will just use hash value of the attribute object to create a color, so that's why the colors appear very random.