Rusty Burlew wrote a great description of how the bees get the pollen into the basket in a recent post on her blog. The pollen basket is filled from below so when pollen collection has only just started the pollen is at the bottom of the tibia.
As the basket fills it holds a surprising amount of pollen without the pellet falling off. I wonder how she knows when it's full?
Honey bees and some solitary bees exhibit flower constancy, that is each bee pretty much sticks to one type of flower at a time. That is confirmed by finding only one colour in the pollen basket.
Bumblebees are more likely to visit different types of flower on one visit, as is easy to see from the multicoloured pollen in their pollen baskets.