In Japan, vegetable dyes have been used for coloring papers from ancient times. The Buddhist scriptures were often dyed yellow with plant dyes, and a lot of colored papers are preserved nowadays in most museums. Spectrophotometric investigation of ancient colored materials provides an effective tool for their characterization. Yet the fading of colors in themselves strongly disturbs an exact identification of individual dyes. In this paper the fading process of Japanese yellow dyes -- Kihada, Kariyasu, Kuchinashi and Ukon -- was investigated in detail by the use of two types of spectrophotometers. The results of the identification of dyes for nine ancient colored papers are also described.