InGaN-based monolithic multi-colour light emitting diodes (LEDs) are studied both experimentally and theoretically with the focus on factors controlling their emission spectra and efficiency. A number of LEDs with different designs of spacers separating active regions providing blue and green light emission is examined. Unexpected behaviour of the multi-colour LED efficiency is explained in terms of a simple balance model, assuming some degradation of materials quality of the green active region grown on top of the blue one. Electrical properties of spacers separating different active regions in the multi-colour LED structures are identified as the major factor controlling the contributions of these active regions to the total emission spectrum. Correlations between the type and level of the spacer doping and the emission spectrum are found by simulations. Alternative ways of the spectral control by using polarization doping in the graded-composition InGaN and AlGaN alloys used as the spacers are suggested. Colour characteristics of blue/green dual-wavelength LEDs are also measured and discussed, regarding their possible applications.