Correct Usage of Grammar

Colour can be spelled with a "u".

The name of the letter u doesn't start with a vowel sound. Would you say:
an united front
an yes man
an useful invention
an universal convention

A lapse in my English skills. Do forgive.

It seems to me that it has more to do with how the word sounds in the beginning rather than the actual letter it starts with.

An umbrella
An underdog
An hour

United, useful, and universe all start with a "y" sound.

So does the name of the letter U (pronounced "you")

Which is why it would be "A 'U'", and not "An 'U'". I guess what I'm getting at is if the word sounds like it would start with a constanant, then "A" would go in front of it, if it sounds like it would start with a vowel, then "An" would go in front of it. I was always under the impression that if a word started with the letter Y, then Y would be considered a constanant and not a vowel.

...This makes much more sense in my head...

It seems to me that it has more to do with how the word sounds in the beginning rather than the actual letter it starts with. An umbrella An underdog An hour United, useful, and universe all start with a "y" sound.

This is the reason i got a C in senior english and an A in spanish...

Comb
Tomb
Bomb

Screw thy language