Higher tiers of craftable mods can be unlocked by unveiling items with said mod. The default background color of the bar is black. Current progress is brown-yellow while progress that will be achieved is highlighted white. Later on white "new" progress in not always garantueed to unlock a complete new tier.
On another note, the unveiled mod does not have to be the same tier as your current progress. It's possible to get mods for selection that are better or worse than the corresponding option on the crafting bench.
- Using a Chaos Orb on an item with veiled mods removes them.