Green fluorescent protein (GFP) variants that carry one extra strand 10 (s10) were created and characterized, and their possible applications were explored. These proteins can fold with either one or the other s10, and the ratio of the two folded forms, unambiguously distinguished by their resulting colors, can be systematically modulated by mutating the residues on s10 or by changing the lengths of the two inserted linker sequences that connect each s10 to the rest of the protein. We have discovered robust empirical rules that accurately predict the product ratios of any given construct in both bacterial and mammalian expressions. Exploiting earlier studies on photodissociation of cut s10 from GFP (Do and Boxer, 2011), ratiometric protease sensors were designed from the construct by engineering a specific protease cleavage site into one of the inserted loops, where the bound s10 is replaced by the other strand upon protease cleavage and irradiation with light to switch its color. Since the conversion involves a large spectral shift, these genetically encoded sensors display a very high dynamic range. Further engineering of this class of proteins guided by mechanistic understanding of the light-driven process will enable interesting and useful application of the protein.