I reported to moo.com that my Christmas cards had printed out too dark, and was emailed back advising that the images needed to be "CMYK colour" rather than "RGB colour" (which was what I'd used). The assistant who'd replied included a link in her email to Moo's help page on print colour, which explained the difference between the two and why an image might appear darker when printed than it does on a computer screen.
What followed was a wrestling match with my paint program, Clip Studio, which didn't want to work in CMYK colour, converted a CMYK image to RGB as soon as it was opened and could only be persuaded to save the image as a TIFF in CMYK colours. When I tried sending that TIFF to moo.com, I was informed that they couldn't print TIFFs and that they would only accept pdfs, pngs or jpegs, the latter two of which aren't CMYK formats.
In the end I had to 1) create a TIFF, 2) upload said TIFF to a website which converted it to a pdf and 3) send that to moo.com. The cards, which moo.com kindly reprinted free of charge, came through the post a couple of days ago and were usable, if still darker than my original images.
Maybe it would be better to work in black and white from now on.