1. Apparatus for step-wise charge control of an electrochromic layer, said layer being in uncolored state for the condition of no charge in said layer, said layer being colored to a first state for a first charge in said layer, and said layer being colored to a second and greater colored state for a second and greater charge in said layer, said apparatus comprising: selectively operable means for selecting a change of layer state to one of said states regardless of the currently existing state of said layer; said selectively operable means including comparator means responsive both to the existing state and to the selected state and having (1) a first control output for the case of the layer in the selected state being more colored than in the existing state (A&lt;B), (2) a second control output for the case of the layer in the selected state being less than in the existing state (A&gt;B); a constant-current charging circuit connected to charge said layer in the event of a signal in said first control output; a constant-current charge-removal circuit connected to remove charge from said layer in the event of a signal in said second control output; first interval-timing means connected for response to said first control-output signal and connected to said charging circuit to determine a timed incremental charge in said layer in the direction of increased color development in said layer, and second interval-timing means connected for response to said second control-output signal and connected to said constant-current charge-removal circuit to determine a timed decremental removal of charge from said layer, the timed interval for a charging increment to produce a given layer-coloring increment under control of said first interval-timing means being greater than the timed interval for a corresponding decrement of charge removal to produce a corresponding color-removing decrement under control of said second interval-timing means, and said decrement of color removal being selected to be slightly in excess of the increment of layer coloring, whereby regardless of the number of selected increment-decrement cycles there will be no cyclic growth in the coloring of said layer.