When you create a k3s cluster using colima it will default to using Docker for the runtime. This means that any Docker image you build or pull will be available to k3s. This greatly simplifies testing locally built images being referenced by Helm charts or Kustomize (or whatever you are using).
This is not a feature unique to colima, but rather a feature of k3s if it is told to use the Docker runtime. You can read more at https://docs.k3s.io/advanced#using-docker-as-the-container-runtime.