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It isn't really clear if you're talking about casting simply for training, i.e. to raise the book/realm skill levels, or casting because you need what the spell does. If it's the latter, like in combat, I find that casting at the low yellow power levels gives the best return on the SP investment; it fizzles occasionally, but rarely backfires, so I usually get the effect of the spell. Casting high-damage combat spells far into the red is asking for trouble. Obviously, if you need the spell to succeed (i.e. in-combat Heal Wounds for a character with 2HP left), cast only in the green.
If you're simply looking for training (cast out-of-combat buff/prot/heal/misc spells, rest, cast, rest,....), cast at the highest level available; so what if it fizzles? Exception: if you're using Charm/Mindread training on NPCs, be aware that some of them will get upset and refuse to talk/trade with you if you cast orange/red and it backfires.
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