Hey Sun, the golden tile will double the value of the word it has been used for to link. The bluish (in fact silver) tile doubles the value of that letter that's on the silver tile. Hope this helps! What about the occasional tile that has different color letters, not just background shade? I've seen blue and green font…

They have to be "blown up" by either occupying a tile that is destroyed as a result of making a word with a crown, or via line blast (the purple specials you get for making longer words).