Fill an area selected using 'Instant Alpha' with a specific color in Photos
To fill color using instant alpha:


Select the area with instant alpha.
Press delete. If asked to save as png click yes. This makes a picture with the instant alpha areas transparent.
Selection all (entire picture) and copy.
Paint a rectangle of desired color over instant alpha areas.
Click paste. This puts back in the picture with transparent instant alpha areas over the color background.