TS-23 "Light Blue" is pretty close. Matching blue can be hard, because blue paint fades rather quickly. A GN car's 1968 BSB paint would be much lighter 10 or 20 years later. (Last BSB car in original paint and lettering I photographed was 1991, so they lasted quite a while.)
GN seemed to be quite good at keeping their Big Sky Blue sharp and fresh on passenger equipment. And sort pretty good on the locos. But they "NEVER" repainted a BSB freight car. After several years--maybe 10 or so--they faded pretty badly. Grain cars especially. If you're modeling 1970, the colors would be pretty fresh. By 1975, nope. And later, noper.
Someone somewhere might have repainted one of these. But NOT very often. And if they were repainted, they likely would have been done in BN green.
Ed
Anyone who's seen the GN in person have a suggestion for a match for Big Sky Blue? I think their "Light Blue" and "French Blue" may be in the running. I bought some cars and the blue doesn't match with the photos I've seen of GN equipment; the supplied colors are more muted.
Thx