In Final Fantasy XIV, you will notice that when you are crafting, the colors of the synthesis may change as you go through your craft. They may also spark or even go “unstable”, which causes an elemental swirl to occur around your synthesis.
The colors indicate the “heat” of the synthesis according to the developers. White represents a cool, controlled synthesis, red represents a hot, unstable synthesis, and yellow is in the middle.
When you synthesize an item on white, it has the highest change to increase progress, and it is very likely to be successful. When you choose a synthesis option when it is red, it is most likely to fail, but if you are successful, you will get a larger than normal increase in quality.
Your synthesis may also flash. The faster the synthesis flashes, the more likely it is to start sparking. Sparking indicates that the synthesis is not very stable and is likely to go unstable; the more sparks, the closer it is to instability. Unstable refers to the elemental swirl which sometimes comes out of an item. When an item goes unstable, if you fail it, sometimes the item will lose a ton of quality or even fail outright, making this a risky state to be in.
However, the developers have stated that a synthesis which changes colors a lot, sparks, or even goes unstable and is then subsequently recovered, really helps the chance to create an HQ item. High-quality items are borne through risky crafts, rather than the safe play.
As a result, colors and sparks are not necessarily good or bad. If you are crafting a synthesis that is very high level compared to you, or you just want to be successful, you want to try to only craft controlled and on white or yellow, and use the “wait” option 1-3 times if it is red or unstable, hoping the color changes or it settles down.
On the other hand, if you want to make an HQ item, you want to use bold synthesis and take risks. You might fail the item, but you need a high quality score and a lot of color changes to get HQ items reliably.