It's exhaust colour - which can be a variety of shades, depending on a number of aspects. The Hurricane Mk IIc exhausts were a steel colour originally, but heat and the elements obviously affected this, as did the area of operations, and to be truly accurate you would need to study good colour photos or the real thing. As a basic guide, paint the exhausts a dark steel colour, which can be mixed from black and silver. When dry, this can be touched in, or dry brushed lightly, with a reddish brown, with the outlets themselves a grey shade.
Here's a pic of one of my Hurricane's exhausts, which may give you an idea of what they looked like. To help you for future projects, try to study as many photos as you can of such details, as this will help to build your 'data base', with practice and experience leading to further improvements in techniques etc.
A look through some of the other members' build threads will also provide a tremendous amount of information on how things are assembled, painted, varying effects created and so on. There is so much information and combined expertise on this forum, that if you were to find a fraction of this in modelling magazines, it would cost a fortune in subscriptions!