Adding Coloured Border to Image

[EasyGoingPat]

I am wanting to post images on Instagram, which has the (in my opinion) ridiculous constraint that all images have to be square. The images I am posting cannot be cropped. Therefore, what I am wanting to do is make them square, with a coloured margin top and bottom, with the original image centered vertically and horizontally on the new canvas.

I have read load of guides about resizing canvases and adding borders but don't seem to be able to find any easy way to do this.

I would be extremely grateful of someone could explain how I might go about doing this, please.

Kind wishes - Patrick