you can use the colours with er and est endings??

Question

this sky is bluer since you are not (google translation)
is better to say: this sky is more blue since you are not??

Answer 1

In English, some adjectives are 'absolute' - that means they may not be compared. Colors fall in this category. If something is blue, it is blue. Period. That defines that it is not red, green, yellow, or any other color than blue. Someting may not be either 'more blue' or 'bluer'. It may be a deeper blue, a lighter blue, a greenish blue, or with any other meaningful adjective.

That is the grammatical purist statement. In normal conversation, people will use 'bluer' and 'greener', etc. This is understood and generally accepted, but should be avoided in formal writng and speech.

Answer 2

Of course you can use the -er -est form, if it’s a short color.
More, most sounds weird to me.
It’s certainly redder, greener etc.
However, with bluer we are unsure about spelling, therefore we might turn to more blue.
When the adjective is longer, according to the rule you have to say: more violet.
Only: when do you ever?
You can also play with other suffixes, like reddish, greenish, etc

You are welcome