Hi Sorin
I think that resulting Google title should always be like this:
Brand, Product Title, Colour, Size
Google pretty much does this.
If you list very few products then you can send them individually with colour and size in the title. Just don't put size and colour as variants as they will appear twice in the Google title!
I have however always found that the way Google concatenates (puts together) title + colour + size as perfectly OK.
Concentrate on optimising the title of the products and descriptions along with on-page content.
Regards
Nigel