def _perform(environment)
      res = ""
      res << @before.perform(environment).to_s if @before
      res << " " if @before && @whitespace_before

      val = @mid.perform(environment)
      if @warn_for_color && val.is_a?(Sass::Script::Value::Color) && val.name
        alternative = Operation.new(Sass::Script::Value::String.new("", :string), @mid, :plus)
        Sass::Util.sass_warn <<MESSAGE
WARNING on line #{line}, column #{source_range.start_pos.offset}#{" of #{filename}" if filename}:
You probably don't mean to use the color value `#{val}' in interpolation here.
It may end up represented as #{val.inspect}, which will likely produce invalid CSS.
Always quote color names when using them as strings (for example, "#{val}").
If you really want to use the color value here, use `#{alternative.to_sass}'.
MESSAGE
      end

      res << val.to_s(:quote => :none)
      res << " " if @after && @whitespace_after
      res << @after.perform(environment).to_s if @after
      str = Sass::Script::Value::String.new(
        res, :identifier,
        (to_quoted_equivalent.to_sass if deprecation == :potential))
      str.source_range = source_range
      opts(str)
    end
def _perform(environment)
      res = ""
      res << @before.perform(environment).to_s if @before
      res << " " if @before && @whitespace_before

      val = @mid.perform(environment)
      if @warn_for_color && val.is_a?(Sass::Script::Value::Color) && val.name
        alternative = Operation.new(Sass::Script::Value::String.new("", :string), @mid, :plus)
        Sass::Util.sass_warn <<MESSAGE
WARNING on line #{line}, column #{source_range.start_pos.offset}#{" of #{filename}" if filename}:
You probably don't mean to use the color value `#{val}' in interpolation here.
It may end up represented as #{val.inspect}, which will likely produce invalid CSS.
Always quote color names when using them as strings (for example, "#{val}").
If you really want to use the color value here, use `#{alternative.to_sass}'.
MESSAGE
      end

      res << val.to_s(:quote => :none)
      res << " " if @after && @whitespace_after
      res << @after.perform(environment).to_s if @after
      str = Sass::Script::Value::String.new(
        res, :identifier,
        (to_quoted_equivalent.to_sass if deprecation == :potential))
      str.source_range = source_range
      opts(str)
    end
Evaluates the interpolation.

 @param environment [Sass::Environment] The environment in which to evaluate the SassScript
 @return [Sass::Script::Value::String]
   The SassScript string that is the value of the interpolation