On the colorants.....be darned certain you are getting non-bleed colorants, Reds and the motor oil can mess things up pretty good because they can bleed terrible.

Regarding doing one color and trimming a piece off to inject another color with, sure thing. I make sure I have a bunch stabilizer in the second color and heat it up to like 365 degrees and keep just a slight pressure on the injector handle for about 12 seconds after the mold has filled and the plunger stops. Not a lot of pressure, just enough to know you're pushing down.

I shoot a lot of small plastics so I mix my batches in 4 and six ounces for the most part. Larger baits call for more plastic so slightly larger batches can be whipped up once you develope some good colors.

Keep a notebook handy and write down every single thing you add to each new color . When you hit on a keeper, keep a second copy in a safe place so you create a reference list that you can go back to to replicate those keeper colors.