What is the core of the problem? Is it that when you have two colors like RGB(0,0,255) and RGB(255,255,0) there is no easy formula to calculate what a mix of t

What is the core of the problem? Is it that when you have two colors like RGB(0,0,255) and RGB(255,255,0) there is no easy formula to calculate what a mix of these two colors would look like in the real world? And therefore one needs a lookup table? And since 16Mx16M is a large number, a complete lookup table is impractical?