I would be interested in a game based around MTG but I don't think there's enough "room" outside of the colour pie for another colour. Wizards tried that around the time of the Time Spiral set - they were initially going to introduce Purple - but eventually went back to the drawing board because there wasn't enough room for it with everything the main colours covered. 

And yeah, Artifacts and advanced technology are already covered by colourless magic and every colour except green is strongly associated with making artifacts. (Primarily white, blue and red.)

Also, for the sake of people who don't really know MtG (I'm sure they exist somewhere!) here's a more detailed breakdown of the colour pie.

[hider=White]White is strongly associated with law, order and justice. White magic is heavily defensive and restricts enemies rather than directly attacking them; common white spells involve healing, protection from harm, bolstering the defensive value of creatures or exiling enemy creatures temporarily. One white creature is usually fairly meh, but a line of them make each other stronger; First Strike and Vigilance make white creatures great defenders. White also has a fair number of spells that affect all players equally, such as board wipes that destroy ALL creatures (not just the opponent's creatures) or spells that temporarily prevent ANYONE from dealing damage.
 
White's ally colours are blue (shared interest in defensive and evasive creatures) and green (shared interest in creature buffing). It's enemy colours are black and red.

White is commonly associated with goodness, but is not necessarily always good. White is also the colour of the tyrant and the lawyer, the people who rule through the letter of the law as much as the spirit. At it's best, white is the colour of heroes, protectors and healers. At it's worst, white is the colour of dictators, bureaucrats and mindless tribalism. 
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[hider=Blue]
Blue is associated with the elements of water and air; it is strongly connected with knowledge, logic and illusion. Blue spells often have to do with gaining card advantage over the enemy (drawing more cards, causing enemies to discard their cards) or countering enemy spells. Their creatures tend not to be very powerful but have some effects that make them hard to target, such as Flying or Hexproof. Blue is also sometimes associated with time travel and mind control. 

Blue's ally colours are white and black (shared interest in deception); it's enemy colours are red and green.

Blue is the colour of wizards and rogues, people who live by knowledge but also deception. It is totally amoral; someone blue is usually motivated by knowledge for the sake of knowledge rather than any other goal. Dissection on living subjects, for instance, is very blue - as is mind control. It is the colour of scholars, archivists and researchers, but also thieves and spies. 
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[hider=Black]
Black is associated with immorality and ambition, of doing what it takes to win no matter the personal cost to oneself. Black magic has a lot of effects that hobble the enemy one way of the other, such as by weakening their creatures, causing them to discard cards or directly sapping enemy life points. Black also works heavily with the graveyard; their creatures might get stronger the more cards you have discarded, or it uses spells that allow you to ressurect dead creatures. For this reason, Black is strongly associated with the undead, especially zombies and vampires. Their creatures commonly have Regenerate to make them more enduring, Deathtouch to represent poison or disease or Intimidate which makes them harder to block.

Black's ally colours are blue and red (shared interest in destructive effects and personal freedom); it's enemy colours are green and white.

Someone who uses black magic might well power their spells with their own life force or pay some other price for greater power. Their creatures (especially demons) might be cheaper to summon than others but have some disadvantage to them such as causing the summoner to lose life points, or to benefit the opponent the longer they are on the field. It is the colour of necromancers, assassins and warlocks.
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[hider=Red]
Red is the colour of aggression, energy and passion. It is strongly associated with the elements of lightning and fire. Red has lots of direct-damage effects that target enemy creatures or even enemy players. Its creatures tend to be fast and strong but lack greatly in defense. Haste is a common Red creature keyword, or Red creatures might be costed slightly cheaper in exchange for having to attack every turn without choice. Red is about overwhelming offense in the short term, but at the risk of burning itself out in the long term. 

Red's ally colours are green and black; it's enemy colours are white and blue.

Red is associated with personal freedom and emotions; someone who uses red magic usually feels things very strongly and lives life to the full. They might use red magic to draw more cards at the price of discarding some of their own cards, use some of their creatures as suicide-bombers or burn through their land base to gain a lot of mana quickly. It is the colour of barbarians, shamans and industrialists. 
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[hider=Green]
Green is the colour of the wilds, growth and nature. For this reason, green tends to have the strongest and most powerful creatures. Green spells make these creatures even stronger, summon even more creatures or remove "unnatural" elements such as Artifacts or Enchantments. Their creatures might fight with overwhelming force or have some way to allow the user to gain more mana. Large green creatures commonly have Trample, which smashes through enemy lines to not just kill enemy defenders but attack the caster behind them. Smaller green ones might have Deathtouch or Reach (which allows them to defend against Flying creatures).

Green's ally colours are red and white; it's enemy colours are blue and black. 

Green is the colour of unthinking force and has relatively little to deal with magic outside of hitting them with creatures. It is Darwinian and respects size and strength, but also encourages personal accomplishment and social growth through tribes. Someone who uses green magic is probably physically powerful but what makes them really dangerous would be all their friends. It is the colour of rangers, druids and shamans.
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