Slight tangent but I have identified the same thing with Chronomasters from the same period. All dials were made by Singer circa 1968/69. Same as the Speedmasters. It appears there was a certain batch of black paint used at this time that is pre disposed to the browning. It is also seen in pre62 models but extremely rare and I believe the radium is the factor in these models. Shown below 2 very rare variations on the normal Croton Chronomaster. Made for US jewelry stores in Texas and Florida and sold 1969/1970.
I am now the proud owner of one of the 4 brown that William posted. I went for the second one from the left which also came with a straight back writing. I took off the nice flat link brace and use it on a Nato (my personal preference).
One of my browns definitely lost much of its varnish due to moisture as it spent years in SE Asia in the navy during the war in Vietnam. The original owner had it serviced and kept the old parts.
Not sure what to call this 145.022-69. It's not black but not brown. It's like looking at a black dial through a dirty yellow filter. (It's slightly more prominent or visible in person.) It's like when the sun starts getting lower in the sky and the colors take on a yellowish glow.
I've seen several other dash 69s that are similar. It is more apparent when sitting in the watch box next to other speedies.