Is this sending the textbox values (0-255) to the arduino, the Rx light is certainly flashing.
I don't know enough about VS to give advice on this. To start with how about just sending a byte with a fixed value to the serial port from VS rather than the value from the slider ? That way you know what you are sending and, therefore, what you expect to receive.
Is this the correct way of doing this, reading as an integer? Or should it be as Byte, Char etc.
Serial data arrives one byte at a time so that is what you need to read.
if(sliderVal >= '0' && sliderVal < '11')
This will not work because you are receiving bytes but testing for chars.
Write a program to wait until there is serial data available then read a byte and print it. If you follow my suggestion to send a known value then you will know if what you are getting is correct. Next step, send a series of known values, receive them and print them. Now you know that what you are sending is what you are receiving. Next step, send the slider value, receive it and only print it when it changes (or only send it when it changes).
Come back and post the Arduino code when you have got that working.
In the meantime can you describe what the relationship should be between the slider value received and the RGB value or are there going to be 3 sliders in VS, one each for R, G and B ?