Is it legal to play The Eternal Rainbows just to destroy a terrain if I'm not opposed?
Eternal Rainbows
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Is it legal to play The Eternal Rainbows just to destroy a terrain if I'm not opposed?
No, sorry. All targeting not prefaced by "you may" is required, which in the case of the current wording of The Eternal Rainbows would be "your [target] opposed Personality".