The last ability of Bound by Moonsilver is an ability of a permanent, not of a spell (it can be activated only if Bound by Moonsilver is on the battlefield [C.R. 112.6]), and Bound by Moonsilver is not a colored spell while it's on the battlefield (including while it's attached to a creature) (compare C.R. 110.1 with C.R. 111.1), so Emrakul's protection from colored spells doesn't affect Bound by Moonsilver in either respect (review C.R. 702.16b-c).
Bound by moonsilver vs emrakul the aeons torn