1) Brides in some places know and expect that men in the wedding party will wear something to coordinate, such as vests, ties, cummerbunds, maybe even colored jackets.
2) Chances are very good that an existing tartan will match the colors she has chosen. If you look beyond registered clan tartans to district and fashion tartans, your choices will be even greater.
3) Making your kilt will be a big deal- assuming you are going to bespeak a traditional kilt- and ordering a custom run of tartan will complicate it by several orders of magnitude. Are you a stock size or close to one? Has someone told you about the many sources of kilts made to stock sizes?
4) This is the most important thing. You have come to a kilt site. While many people here immediately understand the wedding colors idea and many share your desire to make someone happy by wearing a tartan designed just for your wedding, the one thing we ALL agree on is that a kilt ought to be a comfortable garment, not some costume that you will wear stiffly and awkwardly. In order to make sure of that, it is probably good to get a kilt as soon as possible. Whether or not that is the kilt you will be married in is less important. It is the kilt you will learn to wear a kilt in. You will learn about admiring looks, swish and swing, and where your kilt-waist is.

Last, GOOD LUCK. Be patient, loving, kind, adventurous, and dependable, and be patient some more.