How do I invert a colour?

Try this:

uint InvertColor(uint rgbaColor)
{
    return 0xFFFFFF00u ^ rgbaColor; // Assumes alpha is in the rightmost byte, change as needed
}

If you want to change every color, try a rotational function (shifting or adding) rather than a flipping function (inverting). In other words, consider the range of 0 to 255 for each single color (red, green, and blue) to be wrapped, connected at the tips like a circle of values. Then shift each color around the cirle by adding some value and doing mod 256. For example, if your starting value for red is 255, and you add 1, you get 0. If you shift all three colors by 128, you get dramatically different values for every original color in the picture, even the grays. Gray 127, 127, 127 becomes white 255, 255, 255. Gray 128, 128, 128 becomes black 0, 0, 0. There's a photographic effect like that called Solarization, discovered by accident by Man Ray in the 1930's.

You can also do rotational operations on each color (red, green, blue) by a different amount to really mess up a picture.

You can also do rotational operations on hue, shifting the hue of every original color by some amount on the hue circle, which alters all the colors without altering the brightness, so the shadows still look like shadows, making people look like Simpsons or Smurphs for example.

The code for a shift by 128 could look like:

public static Color Invert(this Color c) => Color.FromArgb(c.R.Invert(), c.G.Invert(), c.B.Invert());

public static byte Invert(this byte b) {
    unchecked {
        return (byte)(b + 128);
    }
}

It depends on what do you mean by "inverting" a color

Your code provides a "negative" color.

Are you looking for transform red in cyan, green in purple, blue in yellow (and so on) ? If so, you need to convert your RGB color in HSV mode (you will find here to make the transformation).

Then you just need to invert the Hue value (change Hue by 360-Hue) and convert back to RGB mode.

EDIT: as Alex Semeniuk has mentioned, changing Hue by (Hue + 180) % 360 is a better solution (it does not invert the Hue, but find the opposite color on the color circle)

You can use :

MyColor=Color.FromArgb(MyColor.ToArgb()^0xffffff);

It will invert MyColor.