What does "white on rice" mean?

[Xavierka]

English Language Tests, Intermediate level

ESL/EFL Test #390 "English Slang Idioms (123)", question 6

"If private McCan is late to duty one more time, I am going to be on him like white on .........," First Sergeant Wilson said to his squad leader.

(a) wash
(b) light
(c) paint
(d) rice

English Language Tests, Intermediate level

ESL/EFL Test #390 "English Slang Idioms (123)", answer 6

"If private McCan is late to duty one more time, I am going to be on him like white on rice," First Sergeant Wilson said to his squad leader.

Correct answer: (d) rice
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What does "white on rice" mean?