There are two good methods - which to use depends on your relationship with the molder (meaning how much have you paid them and how much volume you do), and how much time you have.
(There are probably other methods that ‘real’ CMF designers do but I’d wager they are similar to these in intentions.)
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Find out what compounders are preferred by the molder and their resin suppliers. RTP was mentioned above and I’ve had good projects with them. This is how to do it (ideally). Ray mentioned RTP. RTP are good people. You ask your molder to introduce you to RTP (or compounder X), and you get on a plane and sit with their mixing experts for a day or two while they shoot plastic chips. You evaluate the chips in their color corrected booths, and make up sets of matching colors for however many resins you need. The last time I went to RTP (Winona MN, lovely place on the river), we mixed-up nylon, ABS, and polypro in a day for one single color. I’ve done a similar engagement with Clariant out of their Phoenix AZ location. The color guys are really wizards, able to add fractions of a gram of tint material to small batches of raw resin to tweak the color. However they ideally need to start with something, which brings us to #2.
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Find a ‘thing’ - ideally plastic of some sort - that is large enough to handle, examine, and collect spectrophotometer readings from. You make your own calibrated spectrophotometer CIELAB readings, establish your error tolerance (we use delta 1.5), and send the chunk of ‘thing’ to the molder. They will then go through #1 by themselves, sending you back chips when they have achieved their take on a good match. You can visually inspect them and then run their chips through your spectrophotometer to validate. I haven’t had much luck going straight to CIELAB as a method of communication, too much runaround for everyone, and if you had the CIELAB to begin with, you could send a sample.
I haven’t found Pantone paper swatches to be good for spectrophotometer readings. Too small and they have paper-like qualities. The plastic chips never really caught on either - and they are expensive to distribute and you never get them back. The benefit of #1 is you get back on the airplane in the evening with a book of plastic chips and the compounder provides the color data, resin pellets, or compounds to the molder.
ps
The honorable mention method is to sit in rural mainland China, drinking hot, strong, red tea with the factory owner while he and your production representative argue in Mandarin. His workers mold sample parts, bringing them to you while you munch on the ‘organic’ peanuts farmed directly in front of the plastic factory. You hold up the brand new parts with your dreams of world color domination to the window, behind which the dull sky manages a sickly beige, and ascertain how many more molding repetitions the owner and your growing dysentery are willing to put up with. You leave the factory with color samples, and resolve to document the color ‘Qingdao Haze’ once back in front of a laptop.