For the regular Silflor tufts, I use both 6mm and 4mm on 28-32mm models.
Colour-wise, it depends what you're going for. For my arid bases, I used the two-tone late autumn 4mm tufts, and drybrushed them a bit with a pale buff (something like Bleached bone would probably do too). The 4mm late autumn tufts can be seen below (click on image for a larger size):

For more a temperate prairie/field I got a mixture of single- and two-tone spring and early summer tufts in 6mm, which look great on a brown base.
Not tried the buffalo tufts (yet), but am curious to see a good review of them!