head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more execution; Sid Meier's Civilization IV: Colonization demands more variance.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.

Optimization · Pure Optimization
Optimising colonial economy and revolution timing on an unknown map.
Portal 2 leans Optimization (Pure Optimization); Sid Meier's Civilization IV: Colonization leans Optimization (Pure Optimization).
Portal 2 leans clearly harder on execution.
Portal 2
Portal placement requires some precision but timing is lenient.
Sid Meier's Civilization IV: Colonization
Turn-based clicks - no physical layer
Sid Meier's Civilization IV: Colonization leans a little harder on reads, though the gap is small.
Portal 2
No opponent; puzzles are static.
Sid Meier's Civilization IV: Colonization
AI rivals follow mappable tendencies
Sid Meier's Civilization IV: Colonization leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
Sid Meier's Civilization IV: Colonization
Map generation and exploration hide early information
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Portal 2
Solving the puzzle's logic IS the entire game.
Sid Meier's Civilization IV: Colonization
Colony economy, specialists and independence timing are deeply solvable
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.