We have some information on multi-channel images in SimpleITK here:
https://simpleitk.readthedocs.io/en/master/Documentation/docs/source/fundamentalConcepts.html#channels
You are correct that SimpleITK stores RGB or vector images with the channels interleaved. If you have an image of [x,y,z,c], where the last channel is the color, this would be considered a 4D Image in SimpleITK. We have limited support for 4D Images, which includes reading, writing, and using the ExtractImageFilter( 4D->3D), and JoinSeries (3D->4D). If you find something missing to convert 4D images please let us know.
I recently had to convert a 4D image into a 3D vector image: