Yeah I guess we do need silver squares to start (there'd be no incentive to paint a square silver anyway, since someone else would get there first)
Maybe you could make use of the existing properties of the game, such as:
- Receive a clue when you land on a yellow square
- Receive a clue when you land on a square with a tree
- Have a spell that reveals a clue
- etc.
Also an idea: clues could be tradeable. Of course, specify that when you trade away a clue, you don't actually "lose" it, you just give a copy of it to the receiving player.