I have a 1975 Dodge D100 Long bed and I just bought it after it sat for 8 years. The paint looks to be in way too good of condition for it to be original, but considering the rest of the truck is in pretty good condition for its age, having 55,000 miles, and I see no signs that it was repainted, its possible it could be the original paint. Though when I look at a sheet online that shows the 1975 Dodge truck colors, the shade of blue that the truck is isn't on the sheet, so I'm unsure if it was even a possible paint option out of the factory. The sheet I was looking at is attached below, and also pictures of the truck. I'm pretty sure the blue is medium blue metallic. Also, the tarp is over the bed to keep pine needles from sitting in it. Its a very early 1975 truck and the actual year it was produced was 1974.

 

Looks like Dodge B8 Midnight Blue.... See if you can find the body tag and get the codes off the tag and decode the info to find out what color the truck was built with...
Nice looking old truck.... Fix it, drive it, and enjoy it....