This will probably be a shorter post. I just want to blurt out some thoughts before I lose them to sleep.
Leylines, or "threads of magic" are each colored. Each color of magic will have a custom calamity table (from Threshold-Limited Magic). On the higher end of calamities, I'll include options for the caster to gain some kind of taint or corruption specific to the color. These could include disadvantage packages like gaining berserk or blood lust when using violence magic. Once certain corruption thresholds are reached, the caster would become susceptible to whispered suggestions from the gods. Abusing magic could have major repercussions, and most people would be distrustful and suspicious of spell casters. There are 7 threads, one for each of the gods:
I think I may create 3-5 tiers for each color, similar to how the cleric spell lists are done for Dungeon Fantasy, but instead of having each tier be a list of spells the caster can select from, they'll have to buy an entire tier (say 5-10 points for a tier of spells). They'll have to buy each tier in order (tier one before tier two, etc.). Not all spells will be covered, and the colleges in GURPS and GURPS Dungeon Fantasy may be largely ignored.
When casting, each tattoo activated will cause 1 damage ("mana burn," though each color would cause a different type of damage), encouraging maximizing the tattoos of whichever colors the caster plans to use. A fire mage might do a 5-mana tattoo, so they can spend 5 mana for fire spells, and not have to tap into other colors (which would cause them to take more mana burn).
The ability to distort or re-channel another mage's mana back into a leyline would require a trained skill ("re-channel," or hopefully something more evocative), and would also require the same mana burn as a normal casting. Dwarves and Elves doing this re-channeling would not be subject to mana burn if it was a color of magic they have innate access to. With Elves having the combustible disadvantage, I may restrict them from using red magic. Although they can't channel, I may give Adamant Dwarves the ability to re-channel spells directed at them and not require them to suffer mana burn (like a magical ground).
When casting spells that require an opposed roll, the target would use their Power attribute, or a magical defense (maybe something like a block active defense) adjusted by magic resistance or susceptibility.
I think the next step is to hammer out some spell lists and calamity tables. Then review for flavor, adjust racial templates, and start on some play-testing.
I should probably add something here.
Thinking along these lines I am visualizing some cultures where the user is tattooed but does not get to cast any spells but gets other benefits, like changing into a different shape, becoming faster before going into battle; etc
It's already something of a non-traditional magic system (as far as RPGs go). I wouldn't think of spellcasting in the same way as in, say D&D. Weaving spells and buffing or transmuting oneself are just all different uses of magic and not what I'd consider different traditions.