If the backslash is the problem, you can use string function replace to replace the escape character using below expression, which replaces backslash with an empty string. So, if you have a text like "color:green"\ then it will get replaced as "color:green"
@replace(activity('Lookup1').output.firstRow['HtmlOutput'],'\\','')
You can use this expression and assign the output to a variable and use that variable in your pipeline in subsequent activities.
Please try and let us know if that helps.
Thanks.