In general, farmers who grow fruits such as strawberry or tomato in greenhouses are very eager to predict the optimal harvesting time for fruits in advance. Therefore, in order to solve these farmers'' needs, this paper suggests the appropriate color indexes representing a ripeness of fruit images which are essential to develop a system that can automatically determine the degree of maturity of fruit. First, we consider the definitions and properties of three color models such as CIE XYZ, CIE LAB, and CIE LUV that are commonly used to represent fruit images. Second, we examine the color components and the extracting method of color feature vectors that is most suitable to accurately determine the ripening stage of the fruit. From the experimental results, the color components that can accurately distinguish the stages of ripening of strawberry were Y component in XYZ color model, A component in LAB color model, and V component in LUV color model. Also, it can be seen that the average, skewness and kurtosis of color values can be used as color feature vectors effective for determining the degree of ripening of strawberry. Therefore, the results of our study are expected to be useful to develop a system that can automatically determine the harvesting time of fruits.