A lot of tosh is talked by a lot of people about the BWMB !
Yes they penalise you for putting coloured fleeces in with white. Unadulterated white fetches more than white with some grey in it. You can get repeatable dyeing only from dyeing on all white fibre. If there are coloured fibres in there too, you get a very attractive marling or heathering effect, but the bale of wool so contaminated will not fetch as high a price as an all-white bale.
If you have coloured and white sheep, put the coloured fleeces in a different sack, then they can pay you the top price for the grade for your white fleeces. If they are all in together, they have to downgrade the white fleeces because of the contamination with coloured fibres.
Our commercial sheep have particularly excellent fleeces and we usually average £3-£3.50 per fleece. It can be hard to work out your price per fleece because of the way the Wool Board do the payments - each year you get an advance on the clip you submit plus the balance due on last year's clip. They have to do it this way as they don't know what they can pay everyone until they've sold all the fleeces.
A few years back, the advance turned out to have been way too high because global wool prices crashed, so the next year the balance payments were very low. And because there was a strong likelihood that the prices would remain low, the advance payment for the next year was also set low. So in that year, a lot of farmers got a cheque that was risible, and often didn't cover the cost of shearling the sheep. But the BWMB has worked hard since, developing markets for our national clip, and prices are much better now.
I am, as you can probably tell, something of a champion for the BWMB