C#

int valueOne = 0, valueTwo = 0, valueNew = 0;
bool count = true;

private void Form1_BackColorChanged(object sender, EventArgs e)
     {
    // set inital trackbar position, do this only once, when user sets a new BG color
    if (count == true)
    trackBar1.Value = (this.BackColor.R + this.BackColor.G + this.BackColor.B) / 3;
    count = false;

    // Get initial trackbar position value
    valueOne = trackBar1.Value;
 }

private void trackBar1_Scroll(object sender, EventArgs e)
 {
     int bgRed = 0, bgGreen = 0, bgBlue = 0;

     // get amount of change in trackbar value
     valueTwo = trackBar1.Value;
     valueNew = valueTwo - valueOne;

     // apply change amount to original color value
     bgRed = this.BackColor.R + valueNew;
     bgGreen = this.BackColor.G + valueNew;
     bgBlue = this.BackColor.B + valueNew;

     // dont allow > 255
     if (bgRed > 255)
         bgRed = 255;
     if (bgGreen > 255)
         bgGreen = 255;
     if (bgBlue > 255)
         bgBlue = 255;

     // dont allow < 0
     if (bgRed < 0)
         bgRed = 0;
     if (bgGreen < 0)
         bgGreen = 0;
     if (bgBlue < 0)
         bgBlue = 0;

     // apply new settings to form
     this.BackColor = Color.FromArgb(255, bgRed, bgGreen, bgBlue);
 }