Which color works the best to motivate a user to hit Upgrade/Buy

I'd personally lean towards an orange / red, but this would depend on the color palette of the site. There's been some good articles about this, see https://convertkit.com/how-color... I would also consider split testing 2 variants if you're unsure, to test which performs better. Hope this helps.

Following!

I'd say it depends a lot on the button's context and contrast to it's surroundings in conjunction with compelling branding. Visual hierarchy is what your going for and colour is a tool in your tool-belt to achieve it, as is scale and contrast. Squinting is a good way to see what stands out :P Here's a few good articles on that: https://www.interaction-design.o... https://visme.co/blog/visual-hie... There are a lot of good examples where companies use their brand colour palette to determine their CTA buttons. Nike, Amazon, Simba, AirBnB and Boots, I still find the squint test works well for those examples. Also people are used to what they already know, learned behavior, such as links being blue and underlined. Jakob Neilson said that 'users spend most of their time on other sites'. So knowing the market you are going for will also help you find what conventions are best used there. Here's a link to the article I'm referring to: https://www.nngroup.com/articles... Long winded, but I hope that helps. If I had to select a colour from the list above with no context, I'd go blue. Just because, Google search result links.