Repeatability test tells us the condition of the lamp flash system and electronics independent of color accuracy. Didymium filter test tells us the wavelength accuracy of the instrument independent of reflectance color accuracy. When taking a measurement the integrating sphere interior wall mixes and homogenizes the light reflected from the sample. The wall coating is a critical part of the process, as much as the wavelength alignment and repeatability of the measurement process. the Green tile is a single color in the middle of the wavelength range and middle of the linearity range. It does a good job of predicting the general color accuracy performance of the instrument. The full Color Tile readings are done when the first two tests are successful and provides a "stress test" of color accuracy by using tiles that represent a fairly complete non-chromatic and chromatic color range. The tiles Red, Orange and Yellow have the lowest reflectance in the range from 360 to 600nm of all of the tiles in the set. Dirty lenses causing diffusion can affect the low reflectance readings as can sphere wall dulling, or aging.