I have found on my colour head that moving the knobs, it is possible to move the alignment of the head slightly so that the second blast of light makes my pictures ever so slightly unsoft. Or, the head is still vibrating when I start the exposure and it is completely out of alignment. Under the lens filters was my solution. So, if you are going to use the filters in the enlargers head, do so slowly/carefully and give it 5-10 seconds to make sure it is not moving when you start the exposure.