Wow First of all Thanks for trying my tutorial (You're the first one to upload the result ) and thanks again for crediting me so much .

To your questions :
I didnt use mental ray because i never worked with it It's that easy. Same for the Architectural - you can use other materials but i think with Architectural it is easier.

Your Rainbow is almost perfect but the shadows are missing.
(it's not like i want everybody to follow exactly my tutorial. Tutorials exist to give you a start help the rest is up to you but if you got problems with lighting or rendering maybe i can help u with that because i'm much better in lighting and rendering than modelling(i suck in modelling))

So all in all nice rainbow and thank you