Green’s Dictionary of Slang

penguin n.1

[the black and white habit, clothing or racial mix]

(US, mainly juv.) a nun or priest.

[Aus] ‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxviii 10/1: penguin: Nun. So named because of their black and white habits.
[US]R. Price Blood Brothers 76: The goddamn nuns were worse [...] Them fuckin’ fascist penguins is somethin’ else, hah?
[UK]A. Bleasdale Who’s Been Sleeping in my Bed 51: I was there once with Holy Joe O’Malley, our parish priest [...] An’ y’know wha’? The lousy friggin’ penguin lifted up the curtain an’ had a good look at me.
[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 39: Penguins Nuns.
[NZ] McGill Reed Dict. of N.Z. Sl.

In compounds

penguin colony

(US black) an inter-racial colony of African Americans and white couples, who, after marrying in Europe, had been forced to flee Hitler.

D. Burley N.Y. Amsterdam Star-News 14 Nov. 16: The ‘penguin colony’ in W. 147th St. made up of Negroes and whites who married in Europe and who are today refugees until Hitler is pulled down.
penguin marriage (n.)

(US black) a marriage between an African American and a white person.

[US]D. Burley N.Y. Amsterdam News 27 Apr. 13: ‘Penguin’ marriages of course have been the way the ball bounces throughout U.S. history.