I try to keep things moving and pour hot on hot or inject hot on a hot hand pour. If you are seeing some separation using junk plastic it may be from residual oils that have bled out of the plastic with cooking that can cause a weak weld or joint. Most of my baits weld fairly tight and if I have a request for eyes and the bait allows for it I dip right up to where the tail starts with the cover dip. The cover plastic seals any iffy joints.
Something to try is to boot the plastic temperature to around 355 degrees for each of the second or third colors. Another consideration is the plastic formula you are using. The softer the plastic, the easier it is to get good welds with only a couple degrees more of heat in the second and third colors.
When doing laminates, try to do them in one sitting. If you do belly colors and let them cool for an hour or so that's fine. But in as little as 24 hours the plastic can exude enough natural oil to get in the way of decent welds when another color is laid over the top of the belly color. I have noticed this more in the MF plastics than in the Do-It plastic.
The more complex your shooting becomes [needing to keep several colors hot for instance], the more you need to find and utilize ways to meet your needs. This is where real though comes into play and what might be working wonders for one person might end up being a nightmare for another. I have a sand bed electric skillet I'll use if I have to keep a bunch of colors active, but that really takes some getting used to and I do have to proof the temps before each use to make sure things won't go up in smoke. I use cooking oil to proof the temp since its almost identical in viscosity to raw plastic and heats more consistent than water. I bought a quart jug of the oil for a buck at a discount store and pour the oil back in the jug after each use so it'll be around for quite a while. Another way to help keep plastic up to temperature is to "nest" the cup in a heavyterry cloth towel that is wadded up so the cup sits down in a pocket. The more often the cup gets set in the pocket the better the heat retention gets.