I'm looking to purchase a used car. I've been doing online research and saw this webpage http://www.usedcarsmells.com . They talk about exhaust color and smells. Is it true that you could tell a lot about the engine by looking at what comes out of the exhaust?

Yes this is correct, you can get important information from the colour of smoke from the exhaust:

Blue/Gray Smoke: Blue/gray exhaust smoke is an indication of oil burning in the combustion chamber. These are possible symptoms and causes:
Valve Seals: Leaking valve seals will cause blue/gray smoke at startup because oil leaks past the seals into the cylinder after the engine shuts down.
Valve Guides: Excessive clearance between the valve stem and the valve guide allows oil to leak past the gap into the cylinder.
Piston Rings: Worn or damaged piston rings will cause blow-by resulting in blue/gray smoke.
Worn Cylinder Walls: Worn cylinder walls cause blow-by resulting in blue/gray smoke.
PCV System: A stuck closed PCV valve will cause excessive crankcase pressure resulting in blue/gray smoke.
Black Smoke: Black exhaust smoke is an indication of a rich fuel condition. These are possible causes:
Fuel Injectors: A leaking or dripping fuel injector will cause a rich fuel condition.
Fuel Pressure Regulator: A stuck closed fuel pressure regulator will cause a rich fuel condition.
Fuel Return: A restricted fuel return line will cause a rich fuel condition.
White/Gray Smoke: White exhaust smoke is an indication that coolant is burning in the combustion chamber. These are possible causes:
Cylinder Head: A crack in the cylinder head (around the coolant jacket) will cause coolant to enter the combustion chamber.
Engine Block: A crack in the deck of an engine block near the coolant jacket will cause coolant to enter the combustion chamber.
Head Gasket: A damaged or blown head gasket will cause coolant to enter the combustion chamber resulting in white/gray smoke coming from the tailpipe.