Are colorless Pokemon considered a 'type' in regards to Xerneas' "Rainbow Force" attack?
Yes, Pokemon may be of a "Colorless" type.
Source: TPCi Rules Team (2016-01-07)
Will a multitype Pokemon such as Golurk with the "Double Type" Ability count for two different types of Pokemon for Xerneas' "Rainbow Force" attack?
Yes, Rainbow Force checks for the number of types of Pokemon on your Bench, not how many Pokemon there are.
Source: TPCi Rules Team (2016-01-07)