As spokeswoman of the PAH, a grassroots movement campaigning to protect ruined homeowners, she has taken on Spain's banks and politicians, winning an accolade from the European Parliament.
That honour crowned four years of abrasive work in which she stood in protest on the doorsteps of evictees trying to stop the police and bailiffs getting in. Back home, it angered the ruling Popular Party (PP).
"The PP went after me. There were some difficult weeks. They called me a terrorist. I received death threats," she told AFP in a cafe in her native Barcelona.
The European Parliament in 2013 awarded its European Citizen's Prize to the PAH, the Platform for Mortgage Victims, after the movement successfully pushed for Spanish lawmakers to approve a housing reform.
Colau has a motherly manner and soft features, but a sharp tongue for those she blames for Spain's recent years of economic hardship, brought on by the collapse of a housing boom in 2008.
"Up until 2008, Spanish ministers were saying that an apartment was the most secure investment of all," she said.
"It was lies, and the sickest thing of all is that when it all blew up, they accused the people of having lived beyond their means."
Since its founding in 2009, crowds of PAH activists have gathered on the doorsteps of families to block bailiffs and police from evicting them.
The group claims to have blocked about 1,000 evictions and helped another 1,000 people get a roof over their head by occupying empty buildings.
Courting arrest, the PAH's followers have occupied the premises of banks they accuse of abusive mortgage-lending and staged noisy protests outside politicians' homes.
Between 2008 and the peak of the crisis in 2012, a quarter of a million seizure orders were served for properties in Spain, the PAH says. Seven out of 10 of those were to evict people from their principle residences.
Despite this, last year there were 3.4 million homes standing empty in Spain, left over from the building bust, according to the National Statistics Institute.
"The housing problem is crucial. It is one of the main causes of social insecurity, but it is not for a lack of housing," Colau said.
"The resources are there but they are badly distributed."
Spanish law allows banks to pursue a defaulting homeowner to pay back the whole of a mortgage loan even after surrendering the property, if the value of the home has fallen.
The PAH campaigned hard for a law to allow ruined homeowners to write off their mortgage debts by surrendering the property, and to guarantee social housing at affordable rent.
It handed a petition with 1.4 million signatures to parliament, which put the popular motion to a debate.
A version of the reform was eventually approved by the PP-dominated parliament in April 2013, but with so many modifications that Colau and the PAH were left disappointed.
Spanish banks received 41 billion euros ($56 billion) of eurozone loans to protect the country's finance sector from collapse as they try to dispose of the unsold properties they own.
"Those properties should be rented as social housing instead of being left empty and falling in to disrepair," Colau said.
"Now the banks have been bailed out, you would think they might give something back, socially," she added.
"But the banks that were bailed out are the ones carrying out the most evictions."