The simple answer to this question would be: When not to? Colour envelopes, whether they are blue envelopes, orange envelopes or purple envelopes, draw attention to your letter and get it noticed. Even some more stylish brown envelopes can nudge your customers to open before the rest of their post!
Think back to the last time you heard the postman deliver your post, you head to collect the pile of boring white envelopes from the door mat or post box and you see a coloured envelope. Instantly your eyes widen as it gets your attention and as you sift through the pile, your hand stops on the coloured envelope and you bring it to the front. Often the coloured envelope is opened first because our inquisitive human nature simply cannot wait to see what we have been sent.
Coloured envelopes are associated with birthday cards, Christmas cards (think red envelopes or green envelopes), party invitations or something equally exciting. Consequently they bring back feelings of receiving such things through the post which evoke happiness in all of us. So we have established coloured envelopes are capable of differentiating your letter from everyone else's, what else can they do? Coloured envelopes are capable of brightening up someone's day. A usually mundane task of collecting the post is brightened up by the injection of colour. Would a bright yellow envelope or pink envelope make you smile? Even a luxurious cream envelope gets your attention over a plain white one, right? Simple, but effective.
Whether you are a business attempting to get your marketing material noticed or an individual inviting people to an event, coloured envelopes offer you a brilliant way of getting the attention your letter deserves. With such a variety on the market, it would be a waste not to explore them. Give it a go, be creative!