Photo printing, neutral border colour

[Paul H]

I'd like to have a collection of photos which are different ratios printed at the same size, using something cheap like a one-hour printing service.
I don't want to crop them, so was thinking of creating a new jpg in the right ratio with a neutral background surrounding the image.
I have Photoshop Elements to do this, question is what colour to use?  Some years ago I attempted something similar, adding captions, black text in white box, but it threw the exposure off and they came back too dark.  Is there a neutral colour which won't do this?
Thanks

[Ham]

Transparent in png should work well, at a guess

[lissotriton]

Surely any half decent photo printing service would have the option to add borders, instead of cropping?

[Paul H]

Surely any half decent photo printing service would have the option to add borders, instead of cropping?

I'm sure they would, however I'm not using a half decent service, but a cheap one which will be automated. I did see a cheap service online that would do what I wanted, but the timescale is such that I need them tomorrow.

Anyway, I've gone for the sort of light grey that you use to set a light meter, I'll see how it goes.

[fruitcake]

I've done this with the local Kodak Express photo shop in the precinct, a place that charges 50p a print or thereabouts. First time, the colours were a bit off - the system increased the contrast and saturation to some preset level that's popular with consumers - and the colour balance had become skewed. So I asked the guy behind the counter if they could be reprinted without the auto preset. He said sure they could, but don't put them through the kiosk, hand the USB stick to him instead. He ran them off again, this time colours were as I'd set them in my edits. I didn't mind paying another 50p.

[Paul H]

Thanks.
The light grey worked fine, far better than white, it's hard to know how much difference without printing alternatives, but these were good enough for purpose.