Dec 17, 2017: Printed using "Zbra" postprocessing - inserting Tx cmds on regular distances in the GCode. Color bleeding/shifting happens in the infill and inner walls - the splitter is big enough for that - except maybe when it reaches the upmost layers.
"Zbra" is done in VBasic and Excel. Should be converted to C or Python before I find it sharable. If you can tell me how to couple it to the post-processor of Cura you could tempt me to share earlier...

FlipFlop works with steel marbles diam 12.75mm, but preferrably smaller (I should try to increase the cross section diameter with 10mm...?)
Gutter accepts marbles up to 15.5mm.

One thing for sure: nephew has already 3 parts that work good enough to be creative.

Dec 13, 2017
updated with FlipFlop version. Keep Cap in place with 23-25mm M3 bolt and nut. I use one additional nut on top of the 'flipflop' and two underneath to keep 'flipflop' in place. You could print a hollow cilinder too...
Yet to be printed and submitted to child handling ;)

Dec 10, 2017
My nephew is keen to have also a splitter for his marble coaster.
Gave it a try this evening.

For the zebra effect: https://www.thingiverse.com/groups/geeetech-rostock-301/forums/general/topic:26474