How to send data in response to HTTP get request from ESP8266?
@Gerben answered my question in a comment all credit goes to him. I retype it here maybe it helps someone in the future. 

I had to send two new line after setting up the header

//header
client.print("\r\n\r\n");
clinet.print("Hello, I'm comming from ESP!!");


And in jQuery I get it like this:

$.get("http://192.168.4.1/rgb(120,50,60)", function(data,status){   
    var receivedData=data;  //=Hello, I'm comming from ESP!!
});


It is simple but it may help some newbie like me.