Hi. I have just been involved in this for the Risborough and district MRC Okehampton layout.
We found that the clumps were best broken initially by hand if large then dropped into a coffee grinder ( idea care of Gordon Gravett)
it is then best transferred to the static grass dispenser asap. If left, i.e. in an ice cream container until you have a good quantity then it starts to clump again, probably due to static.
If you want to mix colours then you may need to 'grind' them twice, 1st to build a stock of mixed grass, then redone to remove the clumps immediately prior to laying