Legally speaking, the legal effect of a signature has nothing to do with what color pen you use to sign it. Even if you sign with a blue pencil or a red marker, it is still valid, as long as they can be seen or confirmed. It is only for the need of document preservation that it is suggested to sign with ink that can preserve handwriting for a long time. In addition, a single financial voucher is generally required to be filled in indelible ink. But this requirement is not absolute. In fact, any handwriting written in ink cannot be truly "permanently" preserved or cannot be tampered with, so it can only be relative.

Important laws signed by dignitaries in the United States and Europe are generally ballpoint pens with neutral dye ink, mostly with black as the main color to show solemnity, not the kind of Indian ink that cannot be removed. Of course, the documents they signed don't have to be so anti-counterfeiting, and they can be kept in a cool and humid place. In real life, many people in Europe and America only sign with blue ballpoint pens, and even gel pens are mostly blue dye ink. However, at present, the signatures of administrative officials and judicial officials in Europe and America increasingly emphasize the use of pens with blue indelible ink, mainly to facilitate access and prevent copying fraud.