The HiliteAncestors property is currently documented as "internal use only" - so I'm not sure where you got example usage from. I think this property is related to the hilite_system function which is used to temporarily highlight blocks/subsystems (for example, in case of an error). The highlighted colors are therefore not saved to the model.
The more appropriate property for you to use is the block ForegroundColor property. You can either use one of the pre-defined colors: black, white, gray, red, orange, yellow, green, darkGreen, blue, lightBlue, cyan, magenta; or specify custom colors by using RGB values. For example:
set_param(gcb, 'ForegroundColor', '[0.2 0.8 0.4]')
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