It would be relatively straightforward to replace the tab bar with a segment control, or a view containing a series of custom buttons etc.
In this way one could have something that looks like a tab bar but the selected icon’s color could be anything rather than the blue for a selected icon on a tab bar.
However is such a thing permissible or would it be rejected from the app store?
There’s no way to predict in advance what an app reviewer will do on any given day. I’d say if it looks good and doesn’t confuse the user then you’ve got a good shot.