Nandi (bull) is definitely the vehicle of Lord Shiva which symbolizes the Lord gets rid of ignorance and imparts know-how and knowledge to his devotees. There's some confusion around the color of Lord Shiva if he is blue or white. The Shiva Purana says his shade is white but https://lorshivs33344.activablog.com/17204825/an-unbiased-view-of-shiva-images