Color interpolators return strings.

This partially reverts ce6526cfe3b70a8555bbb7b2f235af9c916d34a0. Although this
lacks the occasional convenience of a color instance, it has several advantages:
strings are immutable, so there is no concern of modifying the return value
(#1030); performance is improved; sometimes (e.g., Canvas, IE9 #1371) automatic
string coercion of values is unavailable, so strings are more convenient.