How to explain the distances on the site?

Question

Design

I have a block on the site, and the distance between the blocks is 26px, and the distance inside the block is 35px, I made these distances based on my intuition, but then I thought that this was not right and that everything should obey some kind of rules, a basic grid or something else anything.
And also how can I explain the choice of color, how to explain that I chose pink and not green for the button.
Tell me how can I correctly calculate the indents on the site? what info to read?