The coral blue colour (essentially just a nice light blue) is exclusive to Carphone Warehouse and will be available in its stores or online from 30 June in the UK. You'll need to get your phone directly from EE if you want the silver version, which you can preorder from 23 June. Thankfully, neither company is charging any more for the new colours than the existing S8 colour choices.
Samsung showed off these colour choices at our UK launch event, but so far we've only been able to buy the phone in black and a purplish-grey here. The blue version has already been available in South Korea, so it's nice to see some bright colour choices coming our way. Personally, I'm still waiting on Samsung to announce a hot pink version.