There is of course a difference between purple leaves and purple bud. Lots of growers will leave some extra leaf on the bud if it had turned purple. This can give the herb a not so surprisingly leafy taste to it.
The purple calyxes are the goal if one wants purple bud. I find the GDP tops turn slightly purple on the tips of the calyxes, but it is mostly the leaves that turn purple. This can be seen in the picture posted above by revanche.