The present invention relates to a controller (30) for controlling the light- output of two light-emitting devices (10, 20). The two light-emitting devices are adapted to provide a McCandless effect on an illuminated object (40). The controller (30) comprises a first user interface for a user to set a first color (11) to be output by a first light-emitting device (10) and second user interface for a user to set a desired mixed color (81) of a common area (80) being illuminated by both light-emitting devices (10, 20). The controller (30) further comprises a processing circuitry for determining first lighting parameters resulting in said first color (11) when provided to the first light-emitting device (10), and for determining second lighting parameters resulting in a second color (21) when provided to a second light-emitting device (20). A mix of the first color (11) and the second color (21) provides the mixed color (81). The first color (11) and the mixed color (81) are represented in a color space chromaticity diagram such that they define a line and the second color (21) is determined such that it is located on said line in the color space chromaticity diagram on an opposite side of the desired mixed color (81) relative to the first color (11). Further, the second color (21) is determined by the processing circuitry such that the desired mixed color (81) is achieved when the first (11) and the second (21) colors are mixed.