For example, $\ce{[Cr(NH3)2Cl2]+}$
One isomer is green and the other is violet. Why is this so? The ligands are the same, the oxidation state is the same, and so is the coordination number. The only reason I can think of is that if two $\ce{NH3}$ ligands are trans to each other, they repel the 3d orbitals differently than if they were cis. But I don't know why this would occur.
The explanation given in the other question involves anti-bonding orbitals, and I simply have no idea what these are. I'd like an answer which uses highschool chemistry so that I can understand (albeit perhaps in a flawed way ) whats going on.