preposition "of" after [what shape/colour]

[ansonman]

I have made up the sentences below.

(1) What shape (of) seashells do you collect?

(2) What colour (of) flowers do you like?

Some of my non-native friends think you don't need "of" just like their example "What size shoes do you wear?"

However, my other friends don't agree. They think both sound better with "of". What is your opinion? Thank you.

[Alan]

Inclusion of 'of' here is standard. Colloquially, however, it might often be omitted.