At that time, colour photography existed, but was not as widely popular and affordable as black-and-white. If your wife didn't take it, then she must have got it from someone. Once you track the source steps of the photo as to how it got to your wife.
See if the person at each step might have other photos taken from that day and place.
Unfortunately in this case, we do not know the size of the original print, so we can't deduce the size of the negative.
As othe answers have suggested, the 1950s is strongly indicated. Maybe they took the picture or got it from someone else.
Even general questions about dating a photograph are better framed in the context of an actual photograph.
Are you able to post the mystery-date photograph to a public site and include the link in your question?
In the alternative, perhaps circle me on G+ (see my Genealogy.
One has no markings on the front or rear and nothing to identify the year taken.The top, right and bottom do not have a border and look like they have been cut. The photo is sepia in color and looks like it would crack if rolled/bent. Jayne Shrimpton and Maureen Taylor's books are good references, but they concentrate on the 1850-1950 period. The informal outdoor setting suggests it was taken by an amatuer photographer.I think this print has been cut down from a larger original because prints usually had white borders on all sides.Commercially processed prints tended to come in standard sizes, which is derived from the size of the negative.
Typically commercial processesing produced contact prints (exactly the same size as the negative) and enlargements of the whole negative, which maintain the aspect ratio of the negative.
Those other photos would provide more important clues and maybe even indicate the event (birthday, get-together, location) that the photo was taken and enable you to get a more accurate date.