What does the idiom 'white lie' mean?

[young.]

Test No. incompl/inter-14 "Popular Idioms (1)", question 5

It's all right to tell a ......... lie.

(a) red
(b) white
(c) blue
(d) black

Test No. incompl/inter-14 "Popular Idioms (1)", answer 5

It's all right to tell a white lie.

Correct answer: (b) white

Your answer was: correct
_________________________

What's the meaning of white lie?
When we suppose to use a word with red, white, blue, black?

[Torsten]

Hi,

Please take a look at this post:
http://www.english-test.net/forum/ftopic762.html

TOEIC listening, question-response: How come Mason isn't here?