Color Pattern (color.php/ color.css)

We are working on a .net environment and are unable to use the "color.php" to adjust the template color. In our markup file, we have tried to replace this line:

<link rel="stylesheet" href="css/colors.php?color=ed215e" type="text/css" />

with

    <link rel="stylesheet" href="css/color.css" type="text/css" />

Change color.css in the indicated line:

:root {
--themecolor: #ff0000; /* Change this Color */
}

but this change does nto seem to work.

Is there a solution for this issue, a way to accomplish changing the color partner without using the php file?

Thank you for you assistance.