Bob has his private color and mixture from Alice. When he mixes them up he has the same color as Alice when she mixes up mixture from Bob and her private color.
Eve has only two mixtures but no private color, so she can't produce the same color as Alice and Bob.
Eve here can only see colors Alice and Bob exchang but can't change them (Eve is for eavesdropper), if she can change them (intercept mixture from Bob and send her mixture instead) she should be called Mallory (from man in the middle). In this case we have to assume that Alice knows Bob's mixture - he showed it to her earlier in real life or something - and she won't be tricked by fake mixture Mallory sent her.