I have a day of choosing paint colour ahead of me so need to relax my eyes and find the right light. It’s always a good idea when choosing colour to have a really large paint sample - I paint mine onto lining paper. Too many people paint a square directly onto their walls that’s only about 10-15 cm. How is that possibly going to help? Also it will be a complete pain to paint over!
It’s also worth noting that colour changes when you put multiple coats of paint on. A room needs a minimum of two top coats and that means that your reference sample should be a minimum of 2 coats as well.