When you gain a skill up the message may be in one of three different colors, these are explained below:
Purple
If you are receiving purple skill gain messages the skill you are using gives a guaranteed skill gain each time you use it (while under your limit). There are a number of skills that do this such as weapon skills during combat.
Green
You gain a green skill gain when using a skill that gives random skill gains. When using the right level resource or target with such a skill there is a chance for a skill gain and these will show up in green.
Although the formula may differ typically if your skill is within 25pts of the skill level of the resource or target you are using you will have a chance for a green skill gain, to find the skill level of a resource or target multiply its actual level by 5.
Orange
You gain an orange skill gain if you are in a sweet spot. This may happen if you are gathering resources higher than your natural skill through the use of additional bonuses from items, it may happen with other skills but this is not a documented feature. These orange skill gains work like purple skill gains but are short term and will turn back to green skill gains once your natural skill catches up.