I guess the easiest is to use the arg() method of the ESP8266WebServer class (if you are already using that class - it’s not clear from your code extract)
This parses a standard formatted query string list.
You’ll have to change this line:
client.println(" <a href=\"/HourRed/\" onmouseup=\"location.href=this.href+'ColVal='+sliderHourRed.value;return false;\">");
to yield a URL part in the format:
/HourRed?ColVal=200
Then you can use the sample code here:
There are of course many other ways of doing this.
It is unfortunate for you that your post got caught up in the eternal discussion here about the String class.
Edit:
Incidentally, the arg() method returns a value of type String.