Here, I'm copying a reply I made to a good question.
I'd like to hear others' thoughts on lens color vs. process/metals
Welding lenses come in many different colors; Silver, gold, amber,red, green, and blue.
For MIG, I prefer red.
You're trying to filter out the extraneous colors (the "flare" you see around the arc), and only see the colors important to what you're doing.
Aluminum, for example, flares orange but the puddle has a blue-ish glow. Therefore, a "CoolBlue" lens is a good choice for aluminum.
On MIG mild steel, the puddle is red-yellow, but the gas flare around the weld is blue-green. Red is the best filter for this, especially on dual-shield flux-core.