Is it safe to do closed source code on Google Colab?

It's safe, at least as safe as your private Google Doc is.

No one can access your own private Colab notebooks. And Google has the incentive to make it as safe as possible for their reputation. Because, they need to sell GCP to business.

"Safe" is all about trade offs. It's about how much convenience you're willing to sacrifice for how much security. Closed door without a lock is safer than an open door way, a locked and bolted door is even safer, replacing that door with a 1 foot thick slab of metal is even safer, but is probably overkill unless you're a bank.

Similarly, "Is Colab safe for closed source" depends on what your threat model is. Who is coming after your code? If you're a hobbyist or working by yourself, Colab is probably great.

However, if you're a company with many employees and are worried about hackers tricking your employees into sharing their code, or if you want to ensure notebooks stay secret even from other employees outside a single team, then you may want a more secure notebook platform.

Google made Colab as the platform for students and hobbyists, but they've also made AI Platform Notebooks as the platform for serious enterprise users with the heavier requirements.