test.rb

require 'liquid'

class SetClrCondition < Liquid::Tag
  include Liquid

  def initialize(tag_name, condition, tokens)
    super
    @condition = condition
  end

  def render(context)
    t = Template.parse "{% if #{@condition} %}SET{% else %}CLR{% endif %}"
    t.render context
  end
end

Liquid::Template.register_tag('setclr', SetClrCondition)


@template = Liquid::Template.parse("{% setclr a <> c %}")
locals = Hash[%w(a b c d e f).each_slice(2).to_a]
p @template.render locals