The Morse Code Flag for the number 3 has a yellow background, because yellow is the 3rd colour of the visible spectrum. The three stars represent the three dots while the two stripes represent the dashes.
Coincidentally the pattern of stars and stripes mirrors the flag of Washington DC. The primary difference is that the DC flag has a white background and red stars and stripes. Funny tertiary coincidence is that DC is the also the 3rd official capitol of USA, under the US Constitution.