The invention provides an apparatus for producing video frequency color value signals which correspond to a desired arrangement of vertical or horizontal color bars on a television screen. In the case of vertical color bars the apparatus derives during each line period an identical series of pulses having a multiple of the line frequency. Each series of pulses is applied to a binary counter which provides as output a sequence of sets of binary signals. This sequence of sets of binary signals is translated by a pre-programmed store into a further sequence of sets of binary signals, these latter sets of signals being applied to a digital-to-analogue converter adapted to derive therefrom corresponding sets of color value signals. Each set of color value signals defines a differently colored line segment. The effect of this operation over a plurality of successive line periods is to build up a number of vertical color bars each composed of vertically adjacent line segments of the same color derived from identical sets of color value signals occurring in successive line periods.In the case of horizontal color bars the apparatus operates in a similar manner except that the series of pulses initially produced is at a fraction of the line frequency and is derived once for each field period rather than once for each line period.