There are a couple of reasons why red stickers are used for common seal impressions.
Historically in law, a seal on any legal instrument including a contract which is made under seal have great legal effect recognized by the courts of common law jurisdictions. At that point of time, courts mostly used wax seals and wax would normally come in red. So traditionally, people have been seeing documents affixed with red wax seals and have passed on the perception that red seals is a sign of legally recognized document giving people a sense of assurance and security.
Because of such perception, we mostly see documents in Singapore such as school certificates, share certificates and deeds affixed with red stickers for common seal impressions. Stationery shops, bookstores or common seal suppliers generally order red seal stickers as it is more popular amongst their customers.
With the improvement in technology and in order to safe keep the legal documents more easily, many law firms and companies in Singapore have switched to using red stickers for the common seal impressions instead of wax seals. The red sticker is used to imitate wax seal impressions will allow the seal impressions to how better on photocopies.
There is also a price difference between red common seal stickers compared to gold and sliver ones. Gold and sliver common seal stickers are generally more costly compared to red common seal stickers because of higher material costs to make them.
In a way, the above reasons formed a habit amongst the suppliers and users to use red stickers for their common seal impressions compared to other sticker types.