heya... not really. i ve seen it done in a few videos (EDIT: indoors under earthen floor). they used 1 portland to 9 soil/dirt. but... it depends. you could make a test batch with that ratio.
how big do you wanna make that slab?
slope: i would make posts in the corners and screw 2x4 (straight ones) against it with the desired slope. use a level taped to another 2x4 to test if they re parralel. then use a 2x4 to scrape the concrete by using the fixed 2x4 as guidances.
you could probably add another inch of normal concrete to waterproof it.
colour and mosaic
should work, but test.
i m not sure how long it will last because of moisture wicking up from the ground. concrete with not
enough portland tends to loose it's binder when moist over a long time
EDIT: and you could paint it. there are sealers for concrete. but they re not natural. painting with lime might help a bit, but it would need to be reapplied often. lineseed oil? maybe?
you could pour test slabs and test different ratios and treatments. you could use a mold to pour custom-shaped pavers / stepping stones (this would be a good way to test your mixtures and sealers and leave you with at least a few usable pavers)