What are the coffee cups in the cafe? how many coffee cups should the coffee shop prepare?

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Provide a true story that tells you how to run a coffee shop:

Zhang Mei was originally a full-time wife, but later, because her child was old, she was bored at home, so with the support of her husband, she opened a coffee shop. In the course of the operation, she felt very strange. Every time the coffee with moderate concentration was made through experiments, some people always thought it was too strong, while others thought it was too light.

Zhang Mei was originally a full-time wife, but later, because her child was older, she was bored at home, so she opened a coffee shop with the support of her husband, a professional manager in a large enterprise.

In the course of her business, she felt very strange, because every time the coffee that she thought was moderate in concentration, some people thought it was too strong, while others thought it was too light. Why is that?

Zhang Mei, who had done a little research on color, pondered it and thought that it was possible that human vision had caused an error in color. She was not sure whether it was right, so she invited more than 30 friends and put four cups of coffee of exactly the same concentration in front of each of them, but the colors of the cups were brown, red, cyan and yellow.

In the end, friends rated the exact same coffee differently: the coffee in the cyan cup was "too light"; the coffee in the yellow cup was "just right"; and the coffee cup and red cup were "too strong". And 90% of those who think the coffee in the red cup is "too strong".

As a result, Zhang Meiyi changed all the cups in his shop to red, which not only greatly reduced the amount of coffee, but also left an excellent impression on customers. As a result, there are more and more customers and business is booming.