AThe last time I made BC there were small chunks of butter that didn't mix in completely. I didn't notice it unyil after I had colored it. Why did this happen? Is there anything I can do if it happens again?

ASounds like your butter was not at room temp.

I agree with Mandii.  If your butter is not at room temp when you mix it in it will not be soft enough to squash through the mix properly and you will be left with little lumps of butter

Even if you started w/cold butter, if it is mixed/whipped enough it will soften and mix properly.

Once I have me ingredients I leave the b'cream to stir (not whip) at low speed for at least 5 if not 10 minutes for a super smooth easy to use b'cream consistency.

 I find that if your butter is at room temperature your 'bc' is smoother and creamier all the time.

Thanks everyone! I made another batch this morning and made sure my butter was room temp before using it. Worked like a charm-not a chunk in sight!