1. Apparatus for continuously selective charge control of an electrochromic layer, said layer being uncolored for the case of no charge in said layer and being colored to a progressively increasing state of coloration for progressively increasing charges in said layer, said apparatus comprising: automatic means for limiting the charge deliverable to said layer to the extent of producing a predetermined fully-colored state of coloration and for limiting the charge removed from said layer to the extent of assuring a fully decolored state of coloration, whereby automatic limits are established for a predetermined range of colorability of said layer; selectively operable means for selecting a change of state of coloration of said layer within said range regardless of the currently existing state of coloration of the layer, said selectively operable means including a first key for effecting a more colored state of coloration and a second key for effecting a more decolored state of coloration, a constant-current charging circuit connected to charge said layer in the event of first-key actuation, and a constant-current charge-removal circuit connected to remove charge from said layer in the event of second-key actuation; said automatic means including clock-pulse generator means and an up/down counter, said counter being connected upon first-key actuation for up-counting during periods of first-key actuation, said counter being connected upon second-key actuation for down-counting during periods of second-key actuation, there being an up-counting limit at which first-key actuation is effectively disabled, there being a down-counting limit at which second-key actuation is effectively disabled, and the quantity of charge increment per unit up-count being preselected to be sufficiently greater than the quantity of charge decrement per unit down-count such that the resulting unit down-counted decremental quantum removal of color slightly exceeds the unit up-counted incremental quantum darkening of color.