Many recent observations and numerical simulations suggest that nearby massive, early-type galaxies (ETGs) were formed through a "two-phase" process. In the proposed second phase, the extended stellar envelope was accumulated through many dry mergers. However, details of the past merger history of present-day ellipticals, such as the typical merger mass ratio, are difficult to constrain observationally. Within the context and assumptions of the two-phase formation scenario, we propose a straightforward method, using photometric data alone, to estimate the average mass ratio of mergers that contributed to the build-up of massive elliptical galaxies. We study a sample of nearby massive elliptical galaxies selected from the Carnegie-Irvine Galaxy Survey, using two-dimensional analysis to decompose their light distribution into an inner, denser component plus an extended, outer envelope, each having a different optical color. The combination of these two substructures accurately recovers the negative color gradient exhibited by the galaxy as whole. The color difference between the two components (〈Δ(B-V)〉 ≃ 0.10 mag; 〈Δ(B-R) 〉 ≃ 0.14 mag), based on the slope of the M∗-color relation for nearby ETGs, can be translated into an estimate of the average mass ratio of the mergers. The rough estimate, 1:5 to 1:10, is consistent with the expectation of the two-phase formation scenario, suggesting that minor mergers were largely responsible for building up the outer stellar envelopes of present-day massive ellipticals. With the help of accurate photometry, large sample sizes and more choices of colors promised by ongoing and future surveys, the approach proposed here can provide more insights into the growth of massive galaxies during the last few Gyr.