Hi,

An extended pi system or conjugated system (ie 3 or more conjugated bonds) absorbs visible light. This can be the case for a compound like beta carotene which makes carrots red, or something that contains aromatic rings like melanin that makes your skin tan.

For the case of the MCAT, just remember that if you have enough conjugated double bonds, you start absorbing in the visible light spectrum or UV spectrum (depending on the number of conjugated double bonds). With a compound that does not have conjugated double bonds, you do not observe this absorption (unless it is a metal complex).

The reasoning behind this is that the more conjugated double bonds a molecule possess, the lower the energy difference is between its ground state and its first excited state.

For a singly bonded compound, we often observe it as white in colour because the energy difference between its orbitals is very large. However, when you start getting lots of pi bonding, the energy difference becomes smaller. As E= hc/lambda, you can start absorbing wavelengths of visible light if this energy difference becomes small enough (with enough conjugation). You can for example absorb blue light, yielding the complementary red colour.