void setStrokeColorRgb(int r, int g, int b, [num a = 1])

Sets the color used for stroking shapes. r, g, b are 0-255, a is 0-1.

Source

void setStrokeColorRgb(int r, int g, int b, [num a = 1]) {
  this.strokeStyle = 'rgba($r, $g, $b, $a)';
}