There's a 1001 things that could go seriously wrong on Earth without it affecting Mars, and very few that would affect both.
Gamma ray bursts and something wacky happening to the sun are the only things I can think of were a Mars colony probably wouldn't help.
The list of possible extinction level events on Earth were a Mars colony would help - asteroid collision, supervolcano, nuclear war, sudden rapid climate change (>8C change), airborn virus with long (contagious) symptom free incubation period and extreme mortality rate, + a hundred more we haven't thought about yet.
We need a large colony on Mars. Preferably > 100 000 people.
That's not going to happen this century, BUT - if we keep putting off doing ANYTHING, nothing will ever happen.
We need to start solving some of the problems and create a plan for putting a long term manned research base on Mars.