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The refill was added a special material--Catechol at the end of oil. The Catechol is an important chemical raw material, it can change color when expose to light and air. It also can sublimate. When the pen is running out of oil, the writing will turn to red from black, while the red will turn black after exposing to air for a period of time.
It does not affect the overall results, just to alert the user to prepare a new refill.
The College of Feish Design