Please take a look at the image below. I am trying to find out if there's a "rule/pattern", even something that can be put down in math form, to the selection of a two-color gradient. The required outcome is something that looks like it's made "of the same color" but smooth. So, naturally going from RED to BLUE is distinctively a two color gradient.
Suppose I pick the initial color -- how "far" on the RGB scale should the second color be in order for users to consider it "the same color", but in a different tone that creates the illusion of a smooth and interesting gradient background?
If I map the two colors shown uses on a 2-axis color map, will the "vector" created between the two points representing the colors be measure of how I should be picking a second color?
...or perhaps is everything just "Eyeball it, change until the results are pleasing"?
This is how the gradient looks on an iOS app. It's nice, and smooth.