I'd say the only real artistic justification there is perhaps the cheap suggestion of secondary patterns of color, and the increased 'contrast'/'pop' that can be obtained by varying hue.
There's not really much to it (the process of building/using hue variation) in my experience -- you just try to match the perceptual intensity of the 'inbetween' brightness that you would have if you were using monochrome ramping, and avoid significantly increasing the perceived saturation (don't bother with the HSL/HSV 'saturation' value, it's not reliable enough for this). You could consider laying down a reasonable amount of the 'monochrome' intermediate color before then altering that color. A decent palette editor doesn't hurt, but you can definitely also just tweak RGB values, especially if you are using a proper paletted paint program so that you get immediate visual feedback on color changes.