Most direct-view type color displays have a sub pixel structure such as the RGB-stripe structure. The video data for R and B are produced on condition that they are reproduced at the same position as G but they are displayed on the screen with 1/3 of a pixel separation from the corresponding G pixel. This pixel structure thus entails a convergence error of 1/3 of a pixel. The modified s-CIELAB method was applied to investigate color errors produced by the RGB-stripe pixel structure. The results show that the pixel structure can degrade the image quality even in a viewing distance where the pixel structure is not visible, because the color difference caused by the structure has a low frequency component and the human vision can detect it. The same method was applied to the convergence error and showed that the necessary convergence accuracy is around 1/4 of a pixel.