I revisited the menu logic and realized I missed several cases of black-on-white menu options in Plus42:
In the SOLVER and ∫f(x) menus, there is a black-on-white "=", which takes you to EQN mode;
In EQN mode, when you press CALC to enter SOLVE mode, and the current equation has no top-level = sign, the SOLVE menu will include a black-on-white EVAL option, which evaluates the equation directly, without solving for anything;
In EQN mode, when you press CALC to enter SOLVE mode, and the current equation has a name and a parameter list (e.g. F(X):A*X^2+B*X+C), the parameters from the list are shown in white-on-black, while any variables that are referenced by the function and are not in the parameter list are shown in black-on-white;
In the PLOT->PARAM->XAXIS and YAXIS menus, the STK option is shown in black-on-white, while all the variables referenced by the equation, or MVAR parameters defined by the program, are shown in white-on-black;
In the second row of CATALOG, the white-on-black EQN option shows the list of equation variables, while the black-on-white EQN option shows the list of named equations in EQN mode.
In all but the last of these cases, my intent was to highlight the fact that the menu options in question is not a variable or label, but performs a special operation. The only exception to this design principle is the black-on-white EQN sub-catalog in CATALOG; I used it there, really just as a quick hack, because I couldn't think of a good way to distinguish the two EQN sub-catalogs by name, and I ended up just leaving it that way.