I think you have maybe 2 problems. Am I correct you are trying to tie a split colored bug? If so I believe that you are not pulling the first color tight enough and locking it in place before you tie the second color on top. The other thing is I believe with the style fly you are trying to tie that you want to use the flair method described above over a base instead of spinning. This method will allow you to keep the 2 colors separated top and bottom and not spun together.

This is just my experience with deer hair and by no means perfect.