checkIsLiteral = function (node, validColorFunctions) {
  return cssColors.indexOf(node.content) !== -1 || helpers.isValidHex(node.content) || validColorFunctions.indexOf(node.content) !== -1;
}
Check if the value is a color literal

@param {Object} node - The node we're checking
@param {Array} validColorFunctions - The array of valid color function types to check against
@returns {boolean} Whether the node matches the specified types