You always recolour the first colour of a block. The second colour will always be the same. If the two icy blocks are matched while they have the same colour (both have orange or cyan), they'll be destroyed. E.g. if you slide the lower block (with the first colour orange) into the cyan colour changer, it changes the first colour to cyan. It looks like a normal cyan icy block, but it still has two colours. If you slide it into the orange colour changer after that, it'll become orange and cyan again.
Play Level "Foes and Friends"