How to negate an adjective in the past? How can I negate an adjective in the past?

For example:
这是红色的。It is red. 
这不是红色的。It's not red.

How could I say 'It was not red'? I read somewhere that 没有 only negates action verbs. If that's true, would the sentence in the past be the same (apart from time expressions) as the present?
How do you specify past tense for 是?

From my answer:

You have to remember, unlike in English, we do not have past tense for verbs in Chinese grammar. Both "is" and "was" is written as "是" in Chinese.