There are no "natural color" cameras aboard the Hubble and never have been. The optical cameras on board have all been digital CCD cameras, which take images as grayscale pixels.
Sometimes the color is as natural as possible. However, the color given to the images is not just "artistic embellishment." The images are, indeed, downloaded as black and white, and color is added for a number of different reasons – for example, to show the dispersion detail of chemical elements and highlight features so subdued that the human eye cannot see them.
For more information, read The Meaning of Color on HubbleSite, which explains in detail how color is added to images.
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