Which question could be tested in a scientific manner? What causes some people to be color -blind? Why should people be nice to other people? Why is the color purple better than the color red? What are the best shoes to wear when exercising?

The question 'What causes some people to be color-blind?' could be tested in a scientific manner. This question can be investigated through scientific research by studying the genetic and biological factors that contribute to color blindness. The other questions are subjective and cannot be tested scientifically.