You can pick which ever colour you like. But no matter which one, the old stuff needs removing thoroughly. Red is more money but is rated for 5 years between changes. Blue /Green is rated for 2 or 3 years, but is cheaper.
The internal structure of the coolant rank is made up of loads of small compartments that are impossible to get to physically.
Given that a brand new tank and cap is under £30, it's affordable to buy a new one instead of spending ages cleaning an old one.
When you refill the system, use a good strong blend of coolant and use de-ionised water if possible. It's only a few pounds for 5 litres. It won't introduce lime scale into the system.