Managing Colour when sending to an external Printing Service
The difficulty when you are using an exernal printing service or 'lab' is that you have little or no control over the final stages of colour management. Many operators use a one-size-fits-all, push-button approach. Your chances of getting excellent colour matching from such an operator are very low. Caveat emptor!
If you're serious about excellent colour, try to identify a good service — ask a range of experienced photographers where they get their printing done.
A really good lab may cost a bit more. How much probably depends on the price you put on disappointment over a print that isn't up to expectations.
there are some steps you should take to maximise your chances of getting a good quality print.
1 Manage output size and resolution
Double check that the print service has given you the correct information. Many operators simply specify 300 or 240 dpi regardless of the printer they're using, for no apparently better reason than that those numbers are the default output resolution of many photo applications and "they do not wish to confuse customers".
Make sure the operator knows you have resized and must not do so again.
2 Manage output colours
If you have Lightroom or Photoshop, see if the lab will send you a copy of their print profile, and use it to 'soft proof' your image. This may make it possible to make adjustments to your image to counteract any colour peculiarities of their printer.
3 Convert to the required output space
If the operator wants the file converted to sRGB colour space, double check the reason for this. Remember that converting to sRGB will abandon any deeply saturated colours, especially in the cyan hues, that might otherwise be printed.
If their printer has a bigger colour gamut than sRGB, and if the lab uses properly colour managed equipment, they should be able to interpret a high quality image file encoded in AdobeRGB or even ProPhotoRGB which will ensure you deliver all the colours in your image.
It could be that the lab uses a printer that is genuinely incapable of printing colours outside of the sRGB space. In that case, if you need/want to use that lab, sRGB is your only choice. (Although, if the image you want printed does contain deep cyans you may be better to seek another printer.)
On the other hand, the operator may be suggesting sRGB because, as the 'least common denominator' colour space, it is simply convenient and, again, "they do not wish to confuse customers". In short, check before converting to sRGB and abandoning colours.
Assuming it has supplied it for your soft proofing, the lab may be happy for you to convert your file to the supplied printer profile, so you deliver it 'driver ready'.
In any case, be sure to advise the operator of what colour space your image is encoded in, to avoid these kinds of problems.