It seems very likely that the patterns of colors in the lights on the borders of the envelopes is in some way able to be decrypted into information of some importance to the puzzle. These patterns are listed on the envelopes page.
Because of the fact that encrypted messages have already been part of the puzzle (via the day 1 Caesar cipher), it seems likely that this is a good route to take in deducing the role of the lights in this puzzle. Thus, this theory has been classified as promising.
Importance of the Green Light[]
The fact that all of the envelopes have a green light immediately below the postage makes it seem likely that this is an indicator of the start or end point of the information encoded in the pattern.
Baudot Code Theory[]
The fact that there are four/five identified light colours leads to the possibility that a telegraph style alphabet/encoding system has been used. A popular example is Baudot Code, used recently to great effect by Coldplay on the album X & Y where colours stood in for the bits used.
Seven and five digit encoding Theory[]
The number of lights on the cards need to be split grouped to turn into text, one possibility then is that groups of lights would turn into a letter or number, given 49 and 56 lights then a group of 7 lights would use all the lights in groups of 7. For the two larger envelopes with 85 lights you'd need a grouping of 5. 7 bits is interesting because that can be used to find an ASCII character. 5 also is useful as 5 bits gets enough bits to represent 26 letters of the alphabet.