Try to keep the tail color in the mold and use a single edged razor blade to carefully cut the piece off you want to use, but remaining in the cavity where it was shot. When re-shooting the other color, I like to add some stabilizer to the plastic while its still cooloer, heat it up to about 365 degrees and shoot it only holding the injector pump down with pressure on the new plastic for about 5 or 6 seconds before removing the pump. I think the stabilizer helps to heat the plastic to a slightly higher temp without scorching it and that allows for an easy weld where the two colors merge, but maintaining pressure for a few seconds also helps to get all of the air out of the cavities where the tail piece already is in place. Some of the molds are vented well enough that air isn't a problem. Others don't do so well and the warmer plastic helps move the air out. Some molds just don't like the two-color idea if they are not specifically designed for the multitasking..