I might also add that if you do make people see something that's not there or isn't real, that's crossing the line into the manipulation of free will. Glamour is best seen as an enhancement for features or characteristics already there. Even if you did want to make people think that you've got red hair instead of blond or something similar, it requires a huge strength of will. Of course, there is no guarantee people will still be taken in. That, in fact, is the major vulnerability of glamour spells.
Many people refuse to be affected.
Their will effectively negates yours. Others will know something is wrong and will think you untrustworthy. Generalized glamour is far better than specific glamours. A good glamour is subtle and doesn't tell someone something. This limits the problems I've listed.