In addition - does the car have factory air conditioning? If so the spring stripe color would likely be "brown" which was the highest rated spring offered on the Mustang in 65 / 66 (slightly stiffer than the red striped spring to account for the additional weight of the AC compressor). Soak your springs in Evaporust to see if you can see any evidence of the stripes. That process worked for me. I have a '65 Mustang GT with AC (factory GT and AC - late July of 65 Metuchen build date) and my car had the brown striped springs.
My understanding of the various springs offered on the 65 / 66 Mustang and their markings is listed in the table below primarily using the Master Parts Catalog (which I realize are imperfect).
Part Number Load Markings
C5ZZ-5310-A 1279 lbs. PINK
C5ZZ-5310-B 1344 lbs. GREEN
C5ZZ-5310-C 1490 lbs. GRAY
C5ZZ-5310-D 1413 lbs. VIOLET
C5ZZ-5310-E 1491 lbs. BROWN
C5ZZ-5310-F 1413 lbs. RED
Pete Bush did some nice research on the topic of 1966 Front Springs which he documented in his blog.
http://mustangcodex.blogspot.ca/Regards,
Ron