OK, I'm no rocket scientist or food genius but I think I may have solved a HUGE problem for me.
I always have a problem with the white streaks in my BC. I've read TONS of different possible reasons for this....your water, salt, milk temperature, not sifting PS, butter, not stirring enough, old color, everything in the book. It doesn't matter what I did, I would always get the white streaks in the icing.
Now I don't know why but I have an answer!! It sometimes may be dangerous, but here goes----I made an all crisco icing, mixing the crisco, flavoring and milk (or water) first. I added my color to that (considering that it will be slightly lighter when I'm done) and once that is mixed really well, I added my sifted PS. It turned out GREAT! Just to test my hypothesis (heehee), I even tried a batch right after, NOT doing the color first, and I got the white streaks.
So like I said, I'm not sure why but over the past week I've made a good bit of icing for several cakes and they have all turned out great!
I thought maybe this could help some of you guys out there too!

muh, that's good to know.

What type of color are you using, paste, powder, etc?

Then maybe you were not mixing The colour in thoroughly? It's kind of tricky mixing the colour in before. But if it works for you go with it.

I use the gels, either Wilton or Americolor.
And Cake_Princess, I don't think that it's just the color not getting worked in. I've seen so many other people with this problem. When I searched the forums you get a ton of matches. Just thought this might help someone else!