Magyk refers to the powers of wizards and witches. These include the ability to sense things, and to cause others to forget things. Magyk is implied to be a highly advanced form of technology, but this concept is never expanded upon. The seventh son of a seventh son is said to have extraordinary Magykal powers.
Wizards at the Wizard Tower wear blue Ordinary Wizard Robes, except for the ExtraOrdinary Wizard who wears purple robes instead. Wizards' Apprentices wear green robes.
Occurrence of Magykal Abilities
Most people can perform a few basic spells but in order to do more advanced forms of Magyk or Darke Magyk, a person needs to have talent and training at Magyk.
Magykal talent has a chance of occurring in anybody but being related to other people with the talent will increase that person's chance of having it through inheritance.
Magykal Eye Colours
Eye colour is a mutable quality which is affected by the learning of Magyk and Darke Magyk and whether or not the said person has any ability. Angie Sage mentioned that:
|“||Most people can learn to do a spell or two, but it doesn’t mean they have a Magykal part of themselves. Magyk is a skill that, up to a point, can be taught. But to go further you have to have a natural talent, which Jenna does not have. Only if you have this talent do your eyes turn green.||”|
Wizards' eyes become green when they are exposed to Magyk learning. The Heap Children as an example used to have either blue or grey eyes, but their eyes turned green in colour once they had learnt enough Magyk. ShapeShifters, being wizards, also have green eyes.
Witches' eyes can vary, although the most powerful witches always have blue eyes. Zelda Heap is a good example with her sky blue eyes which indicate that she is a white witch. Warlocks are male witches (a rare breed) so they also have blue eyes. Witches, unlike wizards, can be born with their magical eye colour, or develop it later like wizards.
Using Darke Magyk affects Magykal green eyes and gives them little flecks of black until eventually the green is lost. The same applies to Darke Witches, who often end up with black in their typically blue eyes. 
As mentioned in the quote above, whether or not a person's eye colour will change depends on two things: Their talent for magic, and the amount of Magyk that they are exposed to. Non-magykal peoples' eyes won't change no matter how much they are exposed to Magyk.
Rules of Magyk
The rules and limits of Magyk are not fully explored or described within the series. Each Magyk user contains within themselves Magykal power, which is used and depleted when casting powerful spells. Magykal energy can be stored within objects, such as the Akhu amulet and Hotep-Ra's dragon ring. Magykal energy can also be transferred to another person through touch, or shared while co-casting a spell.
The rules of spellcasting depend on the complexity and power level of the spell being cast. Simple spells can be made up on the spot by a Magyk user, by speaking a phrase aloud. Rhyming phrases seem to work best for impromptu casting. This method is somewhat unpredictable, and relies on careful wording and clear intent.
When a Magyk user creates a spell, they may write it down on charms to be used and sold. This ensures that the spell will work the same way each time it is used. The charm usually works regardless if it is used by a Magykal person or not. The item the charm is created from usually relates to the spell, and the quality often matches that of the spell itself.
More powerful spells are generally much more complex, requiring specific ritual movements, timed chants, encoded keystone words, multiple casters, and/or other requirements. They are often delicate spells passed down in writing from many generations ago, and need to be performed precisely or the whole spell falls apart.