You won't be able to do that as the HTML is currently because those first words would need to be in separate element (say a SPAN tag with a class on them) to change only their color. You can change the whole line like this though:
label[for="pid3"], label[for="pid2"] {
color: red;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.