When a character crafts or gathers an item, there is a chance that this action will increase the skill level of the Profession used. Such an increase is called a skillup.

The chance that a skillup occurs is color-coded. These colors appear in the list of craftable items, and in the tooltip of gatherable items. Exact chances for a skillup are not known. Estimates can be found by experimenting. The only certain values are for Orange (guaranteed skillup) and Grey (no skillup).

Chance of Skillup
Color Chance
Orange 100%
Yellow Often
Green Seldom
Grey 0%

Note that when the maximum skill level is reached for the proficiency you have trained in a particular profession, you will not get any skillups. Be sure to train the next proficiency level in time.