Why is this semicolon in red?

A bit of historic background

Before Mathematica version 6 graphics were produced as a side-effect much as Print works now. In fact you can load this old system using:

<< Version5`Graphics`

Now you get this behavior:

Note that the output is - Graphics - and the plot itself is handled like Print. Since there was often little value in having - Graphics - output it was common to see plot statements terminated with a semicolon, which returns Null which is not written to the Notebook.

In version 6 syntax highlighting of that trailing semicolon was added to remind people that with it no output at all would be produced, since the Graphics object is no longer printed as a side-effect.

It's not a syntax mistake. Consider it a suggestion, FrontEnd tells you: "maybe you want to plot it as it has no much sense now".

Of course it may have sense, it's just a suggestion.

If you take a closer look you will find that storing such Plot makes more sense for FE: