A mobile-to-mobile transaction method allows two mobile smartphones to engage in a private transaction between themselves. The two share the same transaction server using prearranged individual enrollments. These build a dynamic digital image of a colorgram with a selected shape that defines a colorgram matrix boundary. Authentication codes, merchant coupons, product advertisements, and browser uniform resource locator (URL) links for product information and ordering, one-time-password (OTP) seeds, initialization vectors, individual enrollment passwords, or password seeds are embedded and encrypted in each colorgram. The colorgram is sent to a first one of the mobile smartphones for its display to a collocated second one of the mobile smartphones. A transaction is authenticated between users based on a calculated expectation of what should be returned when it provides its own digital image of the colorgram displayed by the first mobile smartphone.