Games should not have preset color options for people with colorblindness. Games almost NEVER do them correctly or offer good enough options. They should just give users full-blown color pickers for whatever gameplay elements require them and let users decide for themselves what colors work best.

The Eye was proven to not be all-seeing or all-knowing. Same with the Ring Wraiths. And Orcs were shown numerous times to be inept guards.

So have an eagle fly Frodo to Mt. Doom on a night with a new moon, above the clouds. There is no way they would be spotted. A curse, while stupid, is the only explanation that really puts this plot hole to rest IMO.