What's your ink set? Is is really just cmyk, or do you have more colors in your ink set, like light magenta, light cyan... ?
Light colours don’t really expand on the colour gamut. It’s actually intented for smoothing out the lower ranges of certain colours so it either doesn’t clip the colour or print it very grainy. Now if your printer uses a hexchrome system, which is not likely, then you’ll be dealing with an output device with a wider colour gamut. These systems mostly use hexachrone orange and green inks. This definitely expands colour gamut if setup correctly.
The best is to check with the print provider and try and stick with them. Consistent colour very much depends on a consistent supplier.
I see a lot of “supply in RGB” remarks quite often, but I don’t agree. For pleasing colours on only photos this advice holds some weight, but if you want accurate screen to print to library results, stick to CMYK.
Oh, and don’t forget a good mid to pro range monitor that is properly hardware calibrated.
Colour matching is a dark art, so don’t get sucked into it too much.