In the change-color settings, you set two values - an upper bound and a lower bound - for the game to randomly assign to the unit if it supports change-color in its shaders. (This also applies to effects, lights, light volumes, lens flares, and anything else that takes color parameters). It's just an alternative way of selecting between them. The flag named "...across the long hue path" in similar tags has a similar function - it causes the random assignment to happen in colors the *other* way around the color wheel.
Take this rough representation, with orange as the lower bound and purple as the upper bound.
|_____LOWER|__________________________|UPPER|___|
Normal blending would assign a color somewhere in the green/blue range between the two colors, but having the "long-hue-path flag" checked would assign it somewhere in the red range between them.
HSV blending is the same idea - if you understand how the HSV color palette works, you can use that flag to get blending that RGB normally wouldn't let you have. For example, blending between yellow and blue, with no green in the middle.
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