Mixed tartans:
The tartan that I am wearing is MacNeil of Barra. The kilt and flashes are Ancient, and the tie is the Modern. After looking at the picture a few times, I am inclined to go with JockScot, and get a solid colour tie.
Don't forget that another necktie option would be a club tie; If I remember correctly, you are a member of a genealogical organization -- Mayflower descendants/SAR, etc.? A crested or even striped club tie is always appropriate to wear with Highland day attire.

T.