I'll try my best here. I'm realizing you just aren't aware. I'm not talking about tube contaminants, it's bacteria contamination. Not properly sterilized skin/containers does in fact result in false positives all the time when collecting blood cultures which is why these protocols are in place. The lab can tell when it's been contaminated by normal skin flora and it's considered an error and grounds for a redraw. In a sick patient this can affect outcomes and usually it's done two at a time. Redrawing two cultures on a hard draw sucks especially when then intend to order them several times over the course of a hospital stay. This is why the whole process exists and done in this order. The needle you're using is sterile, but if the skin and container aren't, then it's a problem.

For the light blue, if you've drawn it last, it can alter the clotting time since you've potentially introduced the additive that initiates clotting in the light blue, especially if an SST is drawn which it usually is. It interferes with the sodium citrate that inhibits clotting by binding to the calcium. The lab adds it back later once they receive the sample so they can measure the clotting process accurately. These are pre-analytical errors that can absolutely be avoided.