Sort of wondering if I should replace the list in the OP with this. I've added some summaries, and examples below the list:
Red is the magic of the body, of animals and healing.
Animals, healing (including dealing with microbes), body manipulation, and shapeshifting.
(Colleges of Animals, Body Control, and Healing; Paths of Health and Form, and animal aspects of Path of Nature; RPM Path of Body)
Orange is the power of Fire and Smoke, of Light and Shadows.
Conceptual Fire magics, light & darkness, and illusions.
(Colleges of Fire and Light & Darkness, illusion spells from College of Illusion and Creation; Path of Cunning, Fire and Light/Shadow aspects of Path of Elements; Light and Fire aspects of RPM Path of Energy)
Yellow covers spells of Air and the Mind, and of fortelling the future and observing the past and present.
Conceptual Air magics, weather, mind magics (telepathy, empathy, mental enhancements, et cetra), mental technology (many skills), and divination.
(Colleges of Air, Communication & Empathy, Knowledge, Mind Control, Sound, and Weather, Warning spells from Protection & Warning; Paths of Dreams and Knowledge, Air aspects of Path of Elements, Weather aspects of Path of Nature; RPM Path of Mind, Electricity and Sound aspects of RPM Path of Energy)
Green sings of the magics of the Earth, of plants and the works of Man.
Conceptual Earth magics, plants, fungi, and material technology (e.g. machines & buildings).
(Colleges of Earth, Food, Making & Breaking, Plant Control, and Technology, creation spells from College of Illusion and Creation, and Protection aspects of Protection & Warning; Paths of Gadgets and Protection, Earth aspects of Path of Elements, Plant aspects of Path of Nature; RPM Path of Matter)
Blue is the magic of Water and motion, and of other planes.
Conceptual Water magics, movement (kinetic energy), distance manipulation, travel (and trade or logistics, if it's over some distance), and dimensional magic.
(Colleges of Gate, Movement, and Water; Kinetic (if any) and Water aspects of Path of Elements; RPM Path of Crossroads, Kinetic aspects of RPM Path of Energy)
Indigo, more than any other colour, speaks to the Spirits, and to Magic, itself.
Spirits of all types (including demons and incorporeal undead like ghosts), making (and altering) magic items, and sensing & manipulating magic in general.
(Colleges of Enchantment and Meta-Spells, Spirit and Demon spells from the College of Necromancy; Path of Spirits; RPM Paths of Magic and Spirit)
Purple is a matter of Fate and Destiny, but also of Death and the Undead.
Probability (luck, up to some forms of wish-granting or cursing), death magics, and dealing with all forms of corporeal undead (zombies, liches, some types of vampire or mummy, and so on).
(Bless and Curse from College of Meta-Spells, Wish spells from College of Enchantment, Undead spells from the College of Necromancy; Path of Luck, Embody from Path of Spirits; RPM Paths of Chance and the Undead)
Some magical effects are best done using multiple colours, or can only be done that way. For example, while someone who has been dead for a very short time (a few minutes) might be brought fully to life using Red magic alone, those who have been dead longer would require a combined Red-Purple effect (to transform a dead body into a living one), along with Indigo if the soul that had previously inhabited that body is not present (and to keep other spirits from entering). Turning men to stone (or reversing that) or transforming plants into animals are Red-Green effects. Making an illusion real would combine Orange with one or more other colours, depending on the illusion - making a phantom that's only 'real' enough to move things around or cause damage by physical impact (but has no substance) would be Orange with a minor Blue effect for kinetic energy. Indigo magic can be used to make an effect stronger (or more precise, or a range of other changes).
The colours can overlap in places. A few examples:
*Being able to perceive 'Fate and Destiny' (probability) does allow you to forecast how likely specific events that you ask about are to occur, but will generally not give you information about something that you don't ask about, or ask about happening at the wrong time (e.g. asking about something that you think might happen in three days, but due to things you don't know, it's much more likely to happen in four or five days). Using Yellow magic for divination, you tend to get more details about the future that would have been if you hadn't asked/will be if no-one interferes, including things you didn't know to ask, but the visions you gain can be highly metaphorical, especially if you're new at this or very out-of-practice.
*Purple magic can increase the probability of specific weather occurring, and Blue magic can create a small fog or move clouds together across significant distances, but Yellow magic is more
efficient for weather magic, requiring less time, effort, and skill from the caster for the same or very similar effects.
*Yellow magic can create illusions in specific minds, but requires more effort the greater the number of minds affected, while Orange magic is limited by the size and complexity of the illusion, not the number of minds observing, and is able to fool cameras and other mindless observers. Yellow magic can also affect the simple minds of animals, though this is often easier with Red magic (as long as you aren't just trying to make the beast smarter and/or better able to communicate with humans), or by combining Red and Yellow.
*Indigo magic can have a very wide range of effects by asking spirits to do them, but you can't always be sure that the spirits really understood what you wanted, nor that they aren't interpreting the letter of the contract maliciously (if you use demons, they certainly are); using Indigo magic to amplify another effect by asking the spirits of that Colour of Magic for help is
usually safer, as is using it to add more power to an effect.
*Major social changes can be guided into being by manipulating the minds of influential people (Yellow magic again), altering the fate of the society (Purple magic writ large), negotiating with and convincing the
genius loci (Indigo magic), or by some combination thereof; less controllable changes can be brought by high levels of
any colour. The greater the change and the larger the population of the society, the harder this would be.
Thoughts?