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I am not sure but I think that image_transport should be able to allow you to subscribe to different topics with different data types. Take a look into compressed_image_transport and compressed_depth_image_transport. They both refer to this tutorial. Its a bit vague, but you should have two different parameters(<base_topic>/compressed/format (string, default: jpeg)). Compressed color images should be on /camera/rgb/image_raw/compressed and compressed depth images on /camera/depth/image_raw/compressed. You should be able to change the format (jpeg or png) for them separately. You can check for the color images if you echo this /camera/rgb/image_raw/compressed/parameter_updates for example.

Also this link might help you with another way of setting the compressed parameter.