function _isValidColorExpr(colorExpr){
            // As RGBColor.js is too generous to validate HEX color expression,
            // Check length of colorExpr if it starts with '#' which means HEX.
            if(colorExpr.indexOf('#') === 0 &&
                !(colorExpr.length === 4 || colorExpr.length === 7)){
                return false;
            }

            var rgb = new RGBColor(colorExpr);
            return (rgb && rgb.ok);
        }
Returns whether given color expression is valid

@param colorExpr
@returns {boolean}
@private