To change the doors on the bus you'll need to google grims Unity Asset Editor and open the sharedassets9 file in your skylines directory. You'll need to find the bus2 texture files(you'll need to edit the main texture plus the colours and reflection texture files) and extract, edit with a paint program like paint net and import using grims UAE tool.
Oh I should mention you'll neenthe latest version known as UAEBETABETA as it works with unity 5(which is what skylines runs on). Older versions can't open unity 5 assets.