Translations:KDevelop4/Manual/Working with source code/186/en
From KDE UserBase Wiki
For variables local to a function body scope, rainbow colors are picked based on a hash of the identifier. This includes the parameters to the function. An identifier always will have the same color within its scope (but the same identifier will get a different color if it represents a different object, i.e. if it is redefined in a more nested scope), and you will usually get the same color for the same identifier name in different scopes. Thus, if you have multiple functions taking parameters with the same names, the arguments will all look the same color-wise.
