It totally depends on what you are modeling.
French Gray (like PRR Brunswick Green) fades terribly- almost to a different color.
French Gray has a slightly brownish tint to it when new. See Atlas' B23-7. That was pretty close. They have also done the U25B in it.
But by the time of CSX, that color had faded. Since I model CSX, I use plain old refer gray (floquil) when I paint Family Lines or even SBD System.
By the days of CSX (July 1986) only two locomotives IIRC would have been in fairly new SBD System paint. And that would be the B36-7s (which have not been made by Atlas (YET!) and the SD50s which Atlas has done. So, in 1986, you could model those as CSX French Gray.
But if you model about 1988 or later then I would go with a gray that has no hint of brown.
A photo may not capture what your eye can see as well... but here are a few shots. My guess is that the shots may not show much of a difference. But here goes.
Stock Atlas locos with the darker paint.
U25B

MP15 Stock- but patched for CSX- renumbered.

custom painted SD45 in darker (fresher) paint. AccuPaint SBD System, I think. This model could represent a loco in SBD System days (1984-1986), or a CSX loco, painted in about late 1985 and photo taken in 1986. (SBD system stopped painting units by early 1986 in anticipation of the up-coming CSX merger.)

Stock Kato U30C. Actually looks good for CSX, a bit light for SBD Sys.

Kitbashed U30C, Floquil reefer gray. Weathered darker. Photo skewed blue slightly.

Kitbashed shorty cab. Floquil Reefer gray. Lightly weathered.

Kitbashed U36C. I think Poly-S waterbased reefer gray. Notice it is a bit lighter. Unweathered.

Kitbashed late GP38-2. I think Poly-S waterbased reefer gray. Weathered lighter. This is supposed to be OLD. Photo skewed slightly red- sorry.

Timeline:
Family Lines paint introiduced some time around 1979 (I think). At least early 80s.
SBD SYStem as a railroad was 1984-1986.
CSX July 1, 1986 - present (though some time around 1993-95, yellow ends started being applied to everything if it did not have yellow already.
So as I began.... it depends on what you want to model timewise.