Правильно?
it was cold today, was not it?
it will cold today, will not it?

В целом то да но есть одна ошибка. В конце предложения мы всегда употребляем совершенную форму вспомогательных и модальных глаголов.
It was nice, wasn't it ?

It was cold today was not it
It will be cold today