A yellow or AN yellow? Which is correct?

“A yellow” is grammatically correct. “A“ is the correct article to use with the word “yellow”. “An yellow” is incorrect and should not be used in English.  

I have a yellow guitar.

There is a yellow flower under the table.

Why is it A yellow and not An yellow?

We use “an” with vowel sounds and “a” with consonant sounds.

The word yellow starts with a consonant sound (j) similar to words like user, young and year.

Remember it is not the first letter that determines if we use “a” or “an”, it is the sound. 

Usually words that begin with a “y” have a (j) sound and therefore use the article “a”.

More examples

We all live in a yellow submarine.

We live next to a yellow hotel.