Show a gorgeous silk streamer black on both sides - then wave it in the air as you get the children to throw colours from their clothes towards you.  Then show the streamer has 'caught' all the colours - it's now multi-coloured...red, yellow, green and purple!
Or do it in reverse - show it coloured to start, wave it and the colours disappear - only to be seen to have appeared on any Magic Painting prop you have and coloured it in.
43” (110cm) long