Why those colors on the Spanish flag?

The Spanish flag is composed of 3 horizontal stripes.
The top and bottom ones are red while the middle one, twice the size, is yellow and holds the Spanish coat of arms.
It was designed during the reign of Charles III of Spain, approximately during the second half of the 18th century.
It has been slightly modified over the years…

The colors of the 3 stripes recall the coat of arms of Castile, Aragon and Navarre.
The colors were also chosen because they are readily visible when posted on a ship.
The middle yellow stripe is twice as high so as to insure enough space for the coat of arms, located to the middle-left.
This symbol includes the Pillars of Hercules, with the royal crown on top and 5 shields in the middle, representing the kingdoms of Castile, Leon, Aragon, Navarre and Granada.
The oval-shaped coat of arms of the House of Bourbon rests in the middle, on top of the shields.
A banner is wrapped between the two Pillars, displaying the text “plus ultra”, referring to Spain’s presence overseas.