As you might noticed, lately I use a lot this kind of envelopes, made of Manila paper. I find this paper simple but beautiful, easy to decorate and also related to sort of old-fashioned mail. But I have never realized there are slight differences in the colour (maybe you cannot perceive it; my picture is not very good, sorry!).
I smiled when I read in the Wikipedia: "Because this paper is generally inexpensive, it is commonly given to children for making child art".