I am currently buliding a Westland Wessex RAF rescue helicopter. As the scheme is bright yellow I've got no idea what colour works to highlight the panel lines without making it look silly. Thanks for the advice if you are able to give me some.

I would use a light gray wash, with controlled application until the build-up is at the level needed

Grey wouldn't be too bad. You may try a tan color.
If you haven't applied yellow yet you could do a pre-shade application. You can do a nice dark black along the panel lines first. Then you paint the yellow. Do it in a few layer of yellow and gradually build up a heavier coat in the middle of the panels.

I'd recommend preshading as well. With a light color such as yellow it's easy to control the effect as you gradually build up the color.
Be careful when airbrushing the preshading though: I've noticed that if you have too dark preshading, the subsequent yellow is likely take on a greenish tint on the first couple of coats. Not a big issue (will be rectified by those further coats, but that diminishes the preshading effect), but worth keeping in mind when spraying with pure black. Keeping the lines a less heayv than with darker colors or using a dark grey or lightened black will prevent this.

Testing different preshading colors beforehand will spare you from any unwanted surprises.

Thanks for the advice. This is the first model I've built for a while due to my new daughter and I'd forgotton about pre shading so I have already done the yellow. I think i'll try a light tan colour and see how it goes.