SO2 in litmus solution

SO2 is a bleaching agent, but NOT all dyes can be bleached by SO2. Litmus is a dye which cannot be bleached by litmus. However, SO2 solution is acidic, and thus SO2 can change blue litmus solution to red without further reaction.
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it is a SOLUTION, the reduced form of dye should be COLOURLESS instead of WHITE

2009-02-18 00:02:39 補充：
SO2有能力將濕的litmus paper漂白

2009-02-21 23:13:53 補充：
The litmus dyes is both litmus paper and litmus solution are of the same type

Paper is itself WHITE
A solution is itself COLOURLESS (NOT WHITE)

When litmus dye is bleached, litmus dye become colourless.
For litmus paper, WHITE + COLOURLESS = WHITE
For litmus solution, COLOURLESS + COLOURLESS = COLOURLESS