<===> input.scss
a {
  &.#{#ff0} {
    color: red;
  }
}

<===> error
WARNING on line 2, column 5 of input.scss:
You probably don't mean to use the color value `#ff0' in interpolation here.
It may end up represented as #ffff00, which will likely produce invalid CSS.
Always quote color names when using them as strings (for example, "#ff0").
If you really want to use the color value here, use `"" + #ff0'.

Error: Invalid CSS after "&.": expected class name, was "#ff0"
        on line 2 of input.scss
  Use --trace for backtrace.

<===> error-dart-sass
WARNING: You probably don't mean to use the color value yellow in interpolation here.
It may end up represented as #ff0, which will likely produce invalid CSS.
Always quote color names when using them as strings or map keys (for example, "yellow").
If you really want to use the color value here, use '"" + #ff0'.

  ,
2 |   &.#{#ff0} {
  |       ^^^^
  '
    input.scss 2:7  root stylesheet

Error: Expected identifier.
  ,
2 |   &.#ff0{
  |     ^
  '
  input.scss 2:5  root stylesheet

<===> error-libsass
Error: Invalid CSS after "&": expected selector, was ".#ff0"
        on line 2 of input.scss
  Use --trace for backtrace.