I have been fortunate enough to have commissioned short length single-width custom weaves on three occasions. The first two were for existing tartan designs that are not normally stocked, and the last for a tartan that I designed. These included a Buchanan Hunting in "ancient" colors, a Buchanan in "reproduction"colors, and my own "Leaf Peeper" design, inspired by my annual autumn pilgrimages to New Hampshire.

As the responses to this thread which appear above mine have covered so many points to consider already, I will mention only a few, here.

The confusion about "Weavers Colors"
With regard to the notion of designing with "weavers colors", allow me to define what that means. First, it does NOT mean that all weavers have the exact thread color shades that you see in a tartan design tool, on your screen. If you select a color that resembles "cherry red", know that each weaver's inventory will include some that look "kinda sorta" like cherry red, while others will look "sorta kinda" like it, while others will be more "it could be called cherry red, if viewed under the indirect incandescent light of a refrigerator lightbulb, in the shadow of a zebra stampede."

What "weavers colors" means is that if you select "cherry red", the weaver will look through their shades of red, and THEY will select what THEY feel most closely matches their interpretation of "cherry red", and what THEY feel is most appropriate for the weave. Although the on-screen color in the tartan designer might be labeled "cherry red" for your convenience (or "mustard yellow", or whatever), these are not the way a weaver (or the wool dyer) labels their colors, and are open to individual interpretation.

How a design is recorded by the SRT, regarding colors
Consider how the designs are actually recorded at the Scottish Register of Tartans! For example, my own Leaf Peeper is recorded for posterity, with the following thread count and color information:

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Note that my tartan is recorded as having "Red" in it. Not cherry red, bright red, dark red, fire engine red, blood red, or any other label. A weaver, working only from the recorded description, would have to make guesses as to which shade of red would work well with which shades of the other "generic" color labels the design is to include.

Now imagine going back to the weaver at a later date for a second order, or going to a different weaver, and hoping the colors between the two orders match. Get thread samples (as stressed by others above) each time, and save a swatch of the resulting fabric to match against thread samples, for any later order!

Registering your design
Finally, if you do decide to design your own tartan, consider registering it with the Scottish Register of Tartans. This is the official government body in Scotland that records known tartan designs. When you get your fabric woven, save a swatch to send to them, where they will preserve the sample of your fabric for their archives. The swatch should be large enough to show one complete iteration of the sett.

Another body to consider registering with (and sending a swatch to) is the Scottish Tartans Authority. Where the Scottish Register of Tartans is an official government agency, the Scottish Tartans Authority is a charity "dedicated to preserving, promoting and advancing Scottish Tartans - their origins, history, manufacture, use and development."

Save any scraps!
The last thing I would like to suggest is that you save any scraps that you can from your woven fabric, as you might find them great for craft making, or for making accessories to wear with your kilt (even cuff links, or flashes if so inclined.) You might also like to mount a sample of your fabric in a frame, along with your SRT certificate of registration! A great source for craft-worthy scraps is the cuttings that come from the waist of the kilt, where all the pleats are gathered. Ask your kilt maker to send you all the scraps!