I think size is more of a decision than colour to be able to use it in the evening. I wouldn't use my 35 as an evening bag but think the 25 is perfect however could be a little small for day use
Etoupe would be a great neutral and it is different from black. Also depends on the leather, a grained leather will look too casual for evening. For a day to night look with a 30cm, I'd probably go for swift or box.