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In the Loess Plateau region of China, it was discovered that the famous green-skinned apple cultivar ‘Granny Smith’ could rapidly turn red following bag removal. In order to clarify the reasons for the color formation, we measured and analyzed the changes in anthocyanins, phenolic compounds and expression of the key anthocyanin biosynthetic and regulatory genes in the colored process of ‘Granny Smith’ apples. The results indicated that two types of anthocyanins (i.e., cyanidin 3- O-galactoside and cyanidin 3- O-arabinoside) could be detected in the skin of ‘Granny Smith’ apples after bag removal. Cyanidin 3- O-galactoside accounted for most of the total anthocyanin contents and significantly accumulated within 7 days, along with the concentrations of chlorophylls and carotenoids significantly decreasing. The concentrations of catechin and procyanidin B2 in bagged apples also noticeably decreased, while the concentration of epicatechin increased. After bag removal, the expression of MdDFR, MdANS and MdUFGT increased sharply within 7 days. The transcription level of MdMYB10 and MdBHLH33 was markedly up-regulated and peaked at 1 day. While in the non-bagged apples, little anthocyanins were found and the expression of those genes showed low levels and remained stable. Our results suggested that ‘Granny Smith’ apples, in the special growing conditions of the Loess Plateau region of China, had the potential ability of coloration. And the higher expression of MdDFR, MdANS, MdUFGT, MdMYB10 and MdBHLH33 genes could enhance anthocyanin accumulation on ‘Granny Smith’ apples. [ABSTRACT FROM AUTHOR]