simple gvimrc color defns lost

I am using newly-installed vim 8.0. My _gvimrc is quite simple but does include various syntax-element color definitions, such as:

hi Constant guifg=DarkRed

But these definitions are being over-ridden by something somewhere in the maze of startup files, such that other colors are being applied.

I do not explicitly invoke anything related to syntax highlighting in either _vimrc or _gvimrc. I have installed no plugins beyond whatever comes in the Vim installation itself.

Why is it that a user's choice of colors would be over-ridden by some file somewhere, one which is presumably meant to *augment* the user's choices, not replace them? And how does one overcome this puzzling behavior?

Thank you for your time.

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