咁講係咪錯？係咪一定要加返was?

"so" (adverb)

We can use "so" for comparison in a negative way, to show "something is not to the same degree as other thing" .

Format: A ... not so ... as B

"as" is a preposition, so B (after "as") is the object of preposition and has to be a noun, pronoun, noun phrase or gerund.

你的句子： It's not warm, but it isn't so cold as yesterday.

In this sentence, "it" compares to "yesterday".

"it" is an impersonal pronoun, respresenting "the weather". When used with present tense, it means "the weather today".

The complete meaning of the second part of the sentence is:
The weather today is not as cold as yesterday's weather.

Therefore, you can't add a verb "was" after "yesterday", by all means, otherwise it'll not be grammatical.

所以， a 和 b 都唔啱。
乜都唔加 就啱曬！

2015-05-04 01:15:45 補充：
a 和 b 都唔啱。
乜都唔加 就啱曬！

2015-05-04 01:43:06 補充：
When "as" is used as a conjunction, then you need to put a verb in the clause after "as".

Example:
You can make a comment or add information about what you have just said, by using "as":

The weather is very warm today, as was yesterday's.
~or~
It is very warm, as was yestersay.

It's not warm, but it isn't so cold as yesterday.= Ok, no need to add anything after yesterday.

Yesterday was cold. Today is not warm but isn't so cold as yesterday.

Errors and corrections:-
(b) suggests that not adding anything at end is acceptable;
eg:-Weather doesn't look so cold as much as (that of) yesterday.
eg:-as warm as---
-----as cold as-----
-----as much as that of---
-----as much as (that of)---can be acceptable
It means as the weather do,used when you are comparing two situations as an adverb.