Russian politician Leon Trotsky (1879-1940) was heckled by workmen in Kiev in 1919, He said the town was like a radish—red (communist) outside, but white inside. “Radish” was cited in print in September 1919 and “radish communist” was cited in 1937.
 
Related political terms include “watermelon” for an environmentalist (green on the outside, red or communist on the inside) and “beefsteak Nazi” (brown or Nazi on the outside, red or communist on the inside).
   
   
(Oxford English Dictionary)
radish, n.
= radish communist n. at Compounds 2.
1919 Times 31 Oct. 11/1   A ‘radish’ is a man who fervently professes devotion to the Communist cause while harbouring a secret longing for its overthrow. Red outside, but white..inside. The epithet was invented by Trotsky.
1996   Hartford (Connecticut) Courant (Nexis) 11 Apr. a1   In Poland, we call communists ‘radishes’—they’re only red on the outside.
 
radish communist  n. a person who professes to be a communist without genuine commitment.
1958   C. Bowles Ideas, People & Peace ii. i. 14   At the Potsdam Conference in 1945..Stalin referred to the Chinese Communists scathingly as ‘radish Communists’. They are red, he said, ‘only on the outside.’
1985   Christian Sci. Monitor (Nexis) 26 Mar. 9   Although the Afghan Communist Party claims 200,000 members..only about 1,000 of these truly believe in communism. The rest are ‘radish communists’.
         
18 September 1919, The Sun (Baltimore, MD), “Russ Reds Barbarians,” pg. 3, col. 5:
“Leon Trotsky, however, had a lively time in Kiev, while addressing the workmen. They heckled him and made him very angry. On leaving he said the town was like a radish—red outside, but white inside.”
(From Stanley Graham Larminic, an English Teacher.—ed.)
 
7 August 1922, Duluth (MN) News Tribune pg. 6, col. 1:
The greatest insult you can offer a Russian Bolshevist is to call him a “radish,” this being the scornful epithet invented by Trotsky to designate those who are red only on the outside.
 
Google Books
The Soviets
By Albert Rhys Williams
New York, NY: Harcourt, Brace
1937
Pg. 68:
They are “radish Communists,” red on the outside but white and decayed within — a drag and deadweight on the Party.
 
Google Books
New Designs for Old Mexico
By Henry Albert Phillips
New York, NY: R.M. McBride and Co.
1939
Pg. 147:
However, Bolshevism has left a Red welt of Leftism. One might say that Mexico is splotched with Red. There are many “radish Communists,” as they wittily call them; “red on the outside, but white in.”
   
15 November 1961, Oakland (CA) Tribune, “Subtle Red Tactics Exposed in Film” by the Rev. Bill Rose, pg. A-7, col. 2:
The boy’s battle with temptation is intensified by the advice of a friend to be like a radish—red or communist on the outside and white and pure on the inside.
 
BBC News
What do radicals and radishes have in common?
The Vocabularist
Words unpicked
21 July 2015
(...)
There is a term “radish communist” - said to have been used by both Trotsky and Stalin - for someone who espouses communism without believing in it.
 
Red outside, white inside. Like a radish - but perhaps not radical.