Custom properties with colour names shouldn't be substituted for hex values

[xzyfer]

Originally reported by @alisamar in sass/node-sass#1624

:root {
  --teal: #444;
}

a { content: var(--teal) }
b { content: (- red) }
c { content: (+- blue) }
d { content: (-green) }
e { content: (+ yellow) }

Ruby Sass 3.4.21

:root {
  --teal: #444; }

a {
  content: var(--teal); }

b {
  content: -red; }

c {
  content: +-blue; }

d {
  content: -green; }

e {
  content: +yellow; }

LibSass 3.3.7

:root {
  --teal: #444; }

a {
  content: var(--#008080); }

b {
  content: -#ff0000; }

c {
  content: +-#0000ff; }

d {
  content: -#008000; }

e {
  content: +#ffff00; }

Spec added sass/sass-spec#889

[nickautomatic]

Until a fix for this is rolled out, is there a temporary solution or workaround at all? eg. is there an older version that didn't have this issue? We were using custom properties (with gulp-sass) a couple of months ago with no problem, but now we get this sort of conversion (--white → --#ffffff) happening each time, which makes me wonder if this issue has crept in recently.