Avoid Touching or Almost Touching Colors
Colors that are not touching but very close can cause registration problems too.
Design with the intention of avoiding the need for trapping. If you are printing letterhead with red and blue elements make sure that the two colors never meet each other. If there is enough space between the two colors then any slight misregistration won’t usually be noticeable.
Sometimes your design may have close or tight registration even though the colors don’t touch. A shift on the press could cause those colors to overlap. Talk to your printer if you are unsure about just how close is too close.