...thanks steff, I got this up and running by packing the required data into a JSON object as you suggested, following the deconz API documentation. 
This is the flow:
[{"id":"a2d6592.5f6dfa8","type":"hue-magic","z":"a26c11c5.cbe52","name":"Night Sky","endless":true,"restore":false,"preset":"003-nightsky","steps":"[{\"delay\":0,\"animation\":{\"hex\":\"#0029ff\",\"transitionTime\":15}},{\"delay\":0,\"animation\":{\"hex\":\"#6ca9ff\",\"transitionTime\":15}}]","x":520,"y":780,"wires":[["e13ac6cb.6b0e78"]]},{"id":"e13ac6cb.6b0e78","type":"node-red-contrib-colorspace","z":"a26c11c5.cbe52","name":"","target":"payload","x":680,"y":780,"wires":[["489e9ae.4565864"]]},{"id":"4bcb9d42.6ccbb4","type":"rbe","z":"a26c11c5.cbe52","name":"","func":"rbe","gap":"","start":"","inout":"out","property":"payload","x":370,"y":780,"wires":[["a2d6592.5f6dfa8"]]},{"id":"99e6ea66.95e218","type":"deconz-output","z":"a26c11c5.cbe52","name":"","server":"a2a0bebe.0f766","device":"group_29","device_name":"â—‹ RGB Wohnzimmer","command":"json","commandType":"object","payload":"payload","payloadType":"msg","transitionTime":"","transitionTimeType":"num","x":1120,"y":780,"wires":[]},{"id":"489e9ae.4565864","type":"function","z":"a26c11c5.cbe52","name":"deconz JSON","func":"if (typeof msg.payload.on !== 'undefined') {\n \n var on = msg.payload.on;\n var hue = msg.payload.hsv.hue;\n var sat = msg.payload.hsv.saturation;\n var bri = msg.payload.brightness;\n var transitiontime = msg.payload.transitionTime;\n \n delete msg.animation;\n msg.payload = {};\n msg.payload.on = on;\n msg.payload.transitiontime = transitiontime;\n msg.payload.hue = Math.round(hue / 360 * 65535);\n msg.payload.sat = Math.round(sat / 100 * 255);\n if (typeof bri !== 'undefined') {\n msg.payload.bri = Math.round(bri / 100 * 255);\n }\n \n return msg;\n\n} else {\n return null;\n}","outputs":1,"noerr":0,"initialize":"","finalize":"","x":880,"y":780,"wires":[["99e6ea66.95e218"]]},{"id":"a2a0bebe.0f766","type":"deconz-server","name":"node-red.example.com","ip":"192.0.2.100","port":"80","ws_port":"443","secure":false,"polling":"15"}]
I'll have to add a routine for randomizing colours in the deconz JSON node, but everything else looks good so far.
Thanks again! 
Stefan