Any Unlock in the Oculus requires either blue, green *OR* red Skill Points to unlock. (You can't use a more expensive skill point to unlock a low-tier item) -- eventually, you'll level up so much that you no longer earn the lower-tier Skill Points, so you'll need to rely on other means (like dungeons, or killing Ars Magica bosses) to gather them. You can see what Skill Point you need to unlock a locked skill just by hovering your mouse over the Unlock - the font colour of the tooltip is what type of Skill Point it requires to unlock.
For reference, the easier bosses (Water Guardian, Earth Guardian) drop Blue Skill Points, the middle-tier bosses (Air Guardian, Nature Guardian) drop Green and the end-tier bosses (.. Ender? Fire?) drop Red. Bosses are by far the easiest way to get Skill Points, especially Blue - which you can VERY easily farm off of the Water Guardian, so long as you watch your health. While he IS the first boss.. he's certainly not a pushover. Forgetting to keep distance can be fatal, considering he somewhat pierces armour.
