I had a look at your ads and I must say, the clothing does appear to be a very dark navy, almost black.
I know monitors vary and in your defence, you did mention navy several times.
I think in any replies, I would point that out-that the word navy was in the title and mentioned in the ad several times so there was no reason for any buyer to assume they were black.
You say the photo they have sent shows a fair degree of difference in colour. I am assuming it was quite a bit lighter navy. You saw the item, which photo is more accurate? Yours shows too dark, is theirs too light or is it pretty spot on? That's something we can't gauge, only you can.
If their photo is pretty accurate, I don't think you owe them a full refund & postage at all because they had no reason to suppose the clothing was black, that's their error. And I'd be pointing that out, as I said.
But you could add that under the circumstances you are prepared to make a partial refund to compensate for their disappointment or else they can make a change of mind return for a full refund, but they will need to pay return postage.
However, in future, get the photos right. I don't mean to sound harsh but the rest of your ad looks so professional that people would expect the photography to be pretty well spot on too. Okay, monitors vary but the colour showing in an ad should be reasonably indicative. I've in the past bought clothing on ebay and been very put out to find on a couple of things that the colour differed a lot from what was shown. (These were private low volume sellers).
If a colour isn't showing up properly in your photos, try a few different places-perhaps outside etc so you have a range of photos to show under different lighting.
