Ultra low flush
While half-flush toilets save water, even this may be more than is needed for flushing away urine. And because urination is much more common than defecation, this is a major use of water in a typical household.
In a European-style toilet, it only takes a very small amount of water to reduce the urine concentration and thus prevent the urine causing odor, and to remove the color, for aesthetic reasons.
The best solution may be to modify the half-flush mechanism to use even less water, rather than creating a third option for toilet users.
This is an idea and has not been extensively tested. However some toilets have a handle or button that allows partial flushing, by pressing only partway, and a smaller amount of water is enough after urination.