Information and value

I have bought a set of 5 Rwanda 500 franc coins from the Olympic games in Beijing 2008.
It is a proof set with 5 coins in a weird metal (niobium 999/1000)
Is it normal that the coins have different colors but the same metal or are they painted ??
and is the metal rare ??

also can someone value these coins - there are only 2008 coin sets made and all in proof

Many thanks

Some pictures maybe this helps

Hi!

It is absolutely normal, this metal change of colour with the temperature! But when the color is fixed, it cannot change!

At 200°C, it begin blue with a contact with air.

thanks  for the infomation  

so every coin must heated at a different temperature

and does anybody know the value of these 5 coins as a set ???