Cake decorating experts please. How long does Sugarflair colour paste last

[boogiewoogie]

after it's been opened? I bought a colour a few years ago that I still have and that would be perfect for dd's Frozen themed cake. I am away from home at the moment, so can't check the date on the pot, so I am wondering whether I should buy a new pot from a really good cake decorating shop nearby.
Also, does colouring butter icing blue turn it slightly green because of the butter? Has anyone ever used icing whiteners to make it white first before colouring in?

Any advice appreciated. Thank you!

For butter icing, you need Superwhite which obviously turns it white, then blue. I keep colours sealed and in the dark, never had one go off.

Thanks Dr Seuss. I got Wilton's Sky Blue in the end as the shop I was hoping to do my shopping in was shut today.