Why is the Virgin Mary always depicted wearing blue and what is the main colour associated with the Roman Catholic Church?

ive seen here in red before in pictures...BUT I AM METHODIST...WE HAVE JESUS ON OUR WINDOW.

The color blue has deep biblical roots.

‘And over the table of the bread of the Presence they shall spread a cloth of blue’ (4:7).

"Speak to the people of Israel, and bid them to make tassels on the corners of their garments throughout their generations, and to put upon the tassel of each corner a cord of blue

So blue is a very important color in the artistic traditions of Christians and has deep spiritual meanings that bring out different attributes of the Blessed Mother. It is a sacred color, one that reminds us of Mary’s faithfulness and her privileged role in salvation history.

https://aleteia.org/2017/06/24/why-is-the-blessed-virgin-mary-always-wearing-blue/

Blue was the symbolic color for Israel.

Speak to the people of Israel, and bid them to make tassels on the corners of their garments throughout their generations, and to put upon the tassel of each corner a cord of blue; and it shall be to you a tassel to look upon and remember all the commandments of the Lord, to do them, not to follow after your own heart and your own eyes, which you are inclined to go after wantonly. (Numbers 15:38-39)

Additionally, Numbers indicated that “the Levites are to ‘spread over [the Ark of the Covenant] a cloth all of blue’ (4:6). And further: ‘And over the table of the bread of the Presence they shall spread a cloth of blue’ (4:7).”

The catholic assembly hijacked that notion and the early Roman painters portrayed their paintings of Mary wearing blue - to make an undeserved association of the Roman assembly with God.

No one can ever say what colors Mary wore. No one knows but those who lived with her. And all her clothing was not likely all of the same color.

1) Why is the Virgin Mary always depicted wearing blue

The classical Western depictions of Mary were established by Renaissance artists.
I don't know why blue is such a common color for her clothing in these depictions (since purple represents royalty). Perhaps blue clothing had a symbolism for the Renaissance artists - for example: perhaps it represented virginity.


2) and what is the main colour associated with the Roman Catholic Church?

As far as I know there is no such thing.
The flag of Vatican City is primarily white and yellow, while the coat of arms of Vatican City is primarily red.
https://en.wikipedia.org/wiki/Papal_armorial#Related_coats_of_arms

Blue was associated with purity in the Middle Ages, so Mary was usually depicted in blue clothing.

Even now, blue still connotes purity or cleanliness. Watch TV ads, etc., closely. Personal care products and cleaning products are often shown in blue backgrounds, packaging and others.

I thought the Vatican flag had only white and yellow, not blue.

About Mary wearing blue, I'll try a personal explanation:
From the Roman period to the 17th century, blue dye was mostly natural indigo imported from India.
The cost made it a luxury product worn by wealth or aristocratic people.

For Enlightenment painters, using their more expensive color was probably a way to honor the "Mother of God" (blue was expensive for them too!).
Then blue became a standard associated with Mary image.

Although Mary is not depicted as a rich person in the NT, early artists made her wear blue!

I'm not sure it is blue, because many in the the realm wear white
And the wife if Jesus is depicted in white and gold
The church if the devil is depicted as wearing scarlet

Mary is no longer a virgin.
She and Joseph had sex and children to gether after Jesus was born.

Matthew 1:24, 25
Then Joseph woke up from his sleep and did as the angel of Jehovah had directed him, and he took his wife home. But he did not have sexual relations with her until she gave birth to a son, and he named him Jesus.