Hi,
Here are a few suggestions :
1/ first, find your settings to do single colour prints very well : no stringing, no hairs, ...
2/ make sure the gantry is precisely 90° with bed : otherwise the 2nd head "lathes" the deposited plastic each time it goes over and it drags stuff and makes those balls
3/ to eliminate oozing, I found a very nice solution that consists in turning off the unused head. Works real well BUT it is not very practical if your colors are not stacked vertically OR you are not a very patient man
4/ S3D and other slicers have purge towers and ooze walls. Both are very useful.