A/B Testing a Checkout Button Change

An e-commerce team wants to test whether changing the checkout button color increases conversions. You have an experiment group and a control group, and you need to decide whether the change is worth launching.

How would you set up the A/B test end to end: define the hypothesis, choose the right metrics and guardrails, determine sample size inputs, avoid bias in assignment, decide how long to run the test, and analyze the results before making a launch decision?