What is the blue pattern in printing? I know that printing needs proofing, but I don't know what the blue pattern is.

Under normal circumstances, the proofing cost of printed matter is relatively expensive, so some customers will choose to print only blue patterns when proofing, so that they can only print out to see the design and effect, rather than accurately print out the specific colors, just like giving you a high-pixel picture, you can print it out with an ordinary printer. If customers are satisfied with the overall effect of the design, they can print in large quantities. The main reason for this is to save costs.