I would have thought Enamel paint. But I wonder if it would be best to rough up the shiny surface first,so it can get a grip. I wouldn't think it would do wildlife any harm when completely dry,as my old Dad's pond was made of breeze blocks,rendered,and painted with whatever he had in his shed. Those were the days when they just did it!!
The whole truth is an instrument that can only be played by an expert.
I've got a pre formed pond. It has steep sides but also shelves for plants. I've also got a thin log that is naturally curved and the middle of it just sits on the water surface and supported/ held down by large cobbles at the sides. So a frog/ toad should be able to get in or out .
I'd not paint an enamel bath. I really can't see it lasting being underwater and freezing in winter. If you don't like it being white, you could line it with black polythene.
You just need to disguise the whiteness ( I'm guessing ) so it doesn't need to be anything expensive and fancy.
A stack of housebricks at one end coming to just about water level will allow things to escape.
My thoughts are that the whiteness will soon become green naturally. Also simply putting a stick in a pond will give creatures a way out if they fall in, until the plant life build a little.