Ed
You have the Dk. Green mats.Midnight Green are almost black in color.
I think the Dk. Green mats look great in your car and define a difference between the carpet.
Do you have the matching rear mats too?
There is no right or wrong answer here,as either color mat was possible to be included with your car when it was assembled,or bought later on over the counter.
Unless you have the build sheet or window sticker showing Midnight Green,I would keep them as the contrast looks good as opposed to one straight color throughout the car.
BTW-These particular mats below,
http://www.ebay.com/itm/1967-1968-1969-C...=p2047675.l2557
were never offered by Chevrolet 45 years ago,but are a modern day fantasy idea someone recently licensed/made.