Just wanted to add that because success on the rainbow orb gives the greatest gains in Quality, the abilities that are often used are:

Byregot's Blessing (Carpentry L45) for 100% quality gain on the next 3 steps.
... followed by Culmination (Weaver L45) to ensure Great Success. Used in combination with Byregot's Blessing on a rainbow orb gives you somewhere between a 90 ~ 130 Quality gain.
... followed by Perfection (Carpentry L36) for Great Success on Careful synthesis... hopefully on another rainbow or red orb during Byregot's Blessing for another big gain in Quality.

Increasing your Control stat will reduce the chance of an element going unstable on a failed action, regardless of the color of the orb.

Or you can play off the instability by having the appropriate Brand of ____ set for another 100 Quality gain when it goes unstable...

But that, IMO, is more of a gamble since the element can go unstable and blow up multiple times (ultimately causing you to fail the synth and negate the Quality gain from using the Brand), or not go unstable at all (in which case you waste an ability slot with the unused Brand).

I tend to only do this when leveling on cheap materials. If I'm working with expensive materials I would much rather go for Control and ensure success of the synth.