Based upon your comment that you can view the live color stream in Photo Booth, I am guessing that you have an Astra Pro sensor, which exposes the color stream via UVC (like a standard webcam). This is in contrast to Astra and other OpenNI-like cameras which only expose color via an SDK.
See threads like:
for more on the UVC color camera and differences between Astra and Astra Pro.
Also, some sample code about accessing Astra Pro color:
We will be releasing an update to the Astra SDK that supports the Astra Pro color stream through the same API, but we're trying to get a release of our skeleton tracking done first. See also: https://3dclub.orbbec3d.com/t/sticky-astra-sdk-resources-updates/184