Nevada

[ClassicHasClass]

I thought it was because the feds were getting on their case about outline shields on freeway signs and they decided they'd rather make the spades green than the freeway shields white.

That was my understanding.  FHWA told CalTrans that they couldn't use outlines of shields on guide signs for US and state routes anymore (IIRC they were already using shields for interstates), so they changed the state route shield to green so the outline and the "shield" would be the same.

I'll find the CHPW volume from 1964 I'm referencing tonight.  It talked about the shield color change fairly extensively.  I don't recall anything about the Federal government being mentioned at all. 

Of course, that could also have been, "well, we were going to do it anyway, so there."

[gonealookin]

Also someone asked me how visible the "inverted US 50 shield was" Here's what my dash cam recorded today.
https://www.flickr.com/photos/moabdave/55277593670/in/dateposted-public/

Heh, you went right past my house there.  I'm not in the new places that have the fence signage and flags, which are going for $1.5M-$2M, but I'm in the older subdivision to the right on that same hillside.  That's why I'll know exactly when the white-on-black US 50 shield disappears.

I think Max wants to look at CHPW March-April 1964, page 11 for the California shield discussion.  They considered blue and gold, similar to county route shields, but thought white-on-green provided better visibility.

[Max Rockatansky]

Yes, that's the correct volume.  It has a signage swap out from US 101A to CA 1 on Pacific Coast Highway on the cover.

I had the cover photo on our Facebook page a couple years ago:

https://www.facebook.com/100063655972258/posts/pfbid0217fapxZB2BiH4AeSvZvaPnibmUZJHBzzaJ1ybXdh82xBQv6VJVNMpuSkyvzNiS53l/?fs=e&mibextid=wwXIfr&fs=e

[Max Rockatansky]

The March/April 1964 CHPW article hitting on the green spade can be found on Page 11.  The article goes over the State Highway Renumbering also:

https://archive.org/details/californiahighwa196465calirich/page/n83/mode/2up

FWIW, I do think the green spade does stand out pretty well in Tule Fog:

99CA1 by Max Rockatansky, on Flickr