As others have pointed out, gray is the result of the Red, Green, and Blue values being equal somewhere between 0 and 1 where black is [0 0 0] and white is [1 1 1].
x = rand(200,4);
y = rand(200,4);
grayColor = [.7 .7 .7];
plot(x, y, 'Color', grayColor)
If you are you used to 255 color values from HTML for example, you can divide by 255 to get a value between 0 and 1. Classic HTML gray is [128, 128, 128].
x = rand(200,4);
y = rand(200,4);
htmlGray = [128 128 128]/255;
plot(x, y, 'Color', htmlGray)
