Abstract The purpose of the paper has been to correlate existing scientific data and methods on light and color in rooms and to apply them to the determination of satisfactory color arrangements. The work is based on 1. (a) The integral-equation method of predetermining helios distributions in rooms, 2. (b) The theory of color harmony. The results are in approximate agreement with the best of modern practice (20) but differ from previous formulations by considering the effect of lighting and by employing a scientific specification of color instead of a set of trade names or similar vague terminology. The application of these methods leads to 25 suggested color harmonies for rooms (Tables II and III). Any of these arrangements (used with proper ceiling lighting) will satisfy the 3:1 helios relation and the other objectives mentioned in Section 2. A satisfactory design requires also a consideration of reflectances and colors of furniture and machines. Suggestions are listed in Tables II, III, and VI. Finally, the question of correlated sets of harmonies is treated. Tables VII and VIII present arrangements that allow an interesting variety of colors in a set of rooms. For any given room, there is a choice of four harmonies, yet the total number of paints and floor materials is kept at a minimum.