It has already been shown that readability of chromatic letters on achromatic background can be predicted by using the existent relationship of achromatic documents between visual factors and readability and by replacing luminance contrast with equivalent luminance contrast found from CIE LAB chromaticity. For the easiness of predicting the readability of chromatic documents, it is necessary to select the color space in which equivalent luminance contrast and color difference have high consistency. It was shown that CIECAM97s has higher consistency than CIELAB and CIELUV color space with equivalent luminance contrast.