Sort:  

But there are only 3 different colours, not 4?

You're right, but I think that is to confuse you. Look at the question again.
Example: Each blue marble should touch both yellow and both red marbles, therefore touching all four marbles of a different colour.

Aaah!! I've got you!

Like this

Now, that's clever.
A solution in another dimension.

I didn't have marbles handy.
Is this the solution you were looking for?

Coin Marketplace

STEEM 0.16
TRX 0.16
JST 0.029
BTC 68542.93
ETH 2454.71
USDT 1.00
SBD 2.54