Sass - Assign variable with another variable

Use css calc() function:

$header-font-color: calc(#{$color_pr1});

From the docs: http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#variable_defaults_

You can assign to variables if they aren’t already assigned by adding the !default flag to the end of the value. This means that if the variable has already been assigned to, it won’t be re-assigned, but if it doesn’t have a value yet, it will be given one.

For example:

$content: "First content";
$content: "Second content?" !default;
$new_content: "First time reference" !default;

#main {
  content: $content;
  new-content: $new_content;
}

is compiled to:

#main {
  content: "First content";
  new-content: "First time reference"; }

Variables with null values are treated as unassigned by !default:

$content: null;
$content: "Non-null content" !default;

#main {
  content: $content;
}

is compiled to:

#main {
  content: "Non-null content"; }

You can define a map:

From the Sass Documentation

Users occasionally want to use interpolation to define a variable name based on another variable. Sass doesn’t allow this, because it makes it much harder to tell at a glance which variables are defined where. What you can do, though, is define a map from names to values that you can then access using variables.

SCSSSassCSS
SCSS SYNTAX
@use "sass:map";

$theme-colors: (
  "success": #28a745,
  "info": #17a2b8,
  "warning": #ffc107,
);

.alert {
  // Instead of $theme-color-#{warning}
  background-color: map.get($theme-colors, "warning");
}