Per Lodestone:

When the glowing light is white, the materials are extremely stable, and the chances of success of synthesis commands are increased. The light may also turn from yellow, to red, to multiple colors flashing rapidly. Though these states represent progressively lower chances of success when executing commands, they also indicate times at which the quality of the synthesis can most easily be enhanced, therefore increasing the likelihood of creating a high-quality item.
Before this guide came out and we were all figuring things out on our own, some friends and I came to the conclusion that the order of stability (from most to least) went: white -> red -> yellow -> flashing. Working from this has worked well for me, but today a LS-mate wanted to know the details, so I decided to look it up on Lodestone to see if my experiences matched up with their instructions. That's when I discovered the quote above.

I've been doing crafting leves since, and reversing my handling of red and yellow to see if I've been wrong all along. However, regardless of the conditions, my success rate seems noticeably lower on yellow than it does on red. Elements destabilize more regularly on yellow than red, and failures are also more common. Am I the only one, or have others noticed the same trend?