| 摘要 |
A coloring system is provided in which an undercolor composition is applied to a substrate to thereby leave a colorless mark. The undercolor composition is then written over with an overcolor composition, thereby causing the undercolor mark to become colored. The overcolor composition may itself contain a colorant. In one embodiment, the color-changing effects are accomplished by employing a dye that is colorless in the presence of high pH and/or a reducing agent, but that becomes colored as the pH is lowered, as the undercolor colorant. In a second embodiment, the color-changing effects are accomplished by employing a dye that is colorless at low pH, but that becomes colored as the pH is raised, as the undercolor colorant.
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