An apparatus generates an image signal in which pixels are encoded in N-bit words which encode at least a luma per pixel. A receiver (201) obtains high dynamic range pixel values in accordance with a first color representation in M-bitwords. A first generator (203) includes the high dynamic range pixel values in the image signal in the N-bit words according to a second color representation. A second generator (205) includes in the image signal an indicator that high dynamic range pixel values are encoded. In some examples, the high dynamic range pixel values may be provided in a segment that can alternatively contain high or low dynamic range pixel values, and the indicator may indicate which type of data is included. The approach may e.g. facilitate introduction of high dynamic range capability into e.g. HDMI systems.