Rule 107.1b says
If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles or sets to a specific value a player’s life total or a creature’s power and/or toughness.

It also has two examples.

Example: If a 3/4 creature gets -5/-0, it’s a -2/4 creature. It doesn’t assign damage in combat. Its total power and toughness is 2. Giving it +3/+0 would raise its power to 1.

Example: Chameleon Colossus is a 4/4 creature with the ability “{2}{G}{G}: Chameleon Colossus gets +X/+X until end of turn, where X is its power.” An effect gives it -6/-0, then its ability is activated. Its power and toughness remains a -2/4 creature. It doesn’t become -4/2.

I have trouble understanding how creatures can get negative power or toughness other than through having an effect specifically setting either to a negative value.
Doesn't a game perform a calculation when determining power of Chameleon Colossus after it gets -6/-0? If it does, shouldn't the Colossus become 0/4 instead of -2/4?

The calculation Chameleon Colossus's ability performs is used to determine the answer to the question “What should the value of X be?”. The value of X determines what the result of the effect will be–will the effect give a bonus of 1? 2? 2048? You cannot know until the calculation is performed.

The calculation being done to determine the answer to the question “What is the power of Chameleon Colossus”, on the other hand, is not determining the result of an effect–the effect in this case already exists and has a known result–its result is “Chameleon Colossus gets -6/-0.” What you're calculating is instead the value of Chameleon Colossus's power, given all the known effects that apply to it, and 107.1b doesn't apply to calculations that determine the characteristics of an object.