In Android Studio, you can create an XML file in the res directory that stores all your RGB values in a central location. If two UI elements use the same color, you use the name of that color. So later, if you want to adjust that color, you only have to do it once, in the XML file. You don't have to adjust each element separately.
I'm using the Interface Builder in Xcode 9.0.1, and it appears I have to use the graphical color selector to copy and paste my RGB value. Surely there is a central repository for color variables.