I wrote a guest blog piece for Maker's Row on lab dips. Check it out on their site at - https://makersrow.com/blog/2018/01/what-are-lab-dips-how-to-get-the-perfect-color-for-your-garment/ or right below!
A lab dip is when a dye house or color lab dyes a few small swatches of your actual quality fabric or textile trim to find out what dye formula creates the best color match for that quality. A Pantone color reference or sometimes a swatch cut from another fabric is given to the lab to match to. This process usually takes anywhere from a few days to two weeks depending on how difficult the color is to match, the availability of the dye stuff, and how busy the lab is. At the lab level, it can take up to six hours to process a cotton dip and twelve hours for poly or poly blends. The process is repeated until a close match is achieved. The swatches are then checked in a lightbox to see how colors look under various light settings, like daylight vs. fluorescent store lights. A lightbox is all grey inside so that your eyes do not get distracted by any other colors.
I highly suggest approving lab dips before dyeing bulk or even small lots of anything. A shade of purple looking ever so slightly red might be acceptable to one person and not to another. Color can be very subjective. Most labs will typically send three or four similar looking shades of your lab dipped swatches for review and get your approval before proceeding. If you are working with a very reputable lab or manufacturer, you can often trust the lab to do the matching for you.
Some colors are trickier than others to match too. Blacks tend to be the most difficult as they can ‘flare’ in many directions - like a reddish black or a blue black or a yellow black, etc. Nudes and greys can be difficult as well. If you are lab dipping for something like bras, please give yourself plenty of time to color match all the parts. Getting all the parts to match on a nude bra is difficult. Other items like a hoodie where there is only the main fabric, and maybe a rib trim or drawstring, are much simpler and can take less time.
We also recommend asking for a color card for any yarn, trim, or fabric you are using. Mills will often produce a product in a few basic colors and maybe some trendy ones. Picking an available color can save valuable production time and cost. Or, you could ask what colors the fabric has already been dipped in to see if you can use an existing formula. This is also a good way to go if you are working with smaller units. There are some great dye houses here in the US that can do custom small batches, however.