Buff feathers are coarse in appearance because they are wider and longer than yellow feathers. They tend to look untidy even when the bird is in condition. They can make a budgerigar appear much larger than it really is.
An intermediate feather is a combination of both yellow and buff and is preferred for show budgerigars as it gives the appearance of a larger birds while still retaining good condition and colour.
The Yellow feather is a smaller feather and the colour is more intense and colour in the feathers goes right up to the edges of the feather.