Paul,
A quick capture of some targets at Hancock, Romley and Alpine Tunnel between 1882 and WWI.

At Hancock: Left is 1882 with target tucked against the telegraph pole; right is DL&G era.

At Romley: Left is earlier C&S; next is ca 1891; rightmost image is at east bridge approach around 1900.

At Alpine Tunnel L to R: Late 1880s or 1890s; ca 1900; DL&G; same ca 1900 closer up.
What I see are many in a mid-gray, suggesting a light red, maybe orange, but the emulsion could offer a lot of other choices (white, yellow and blue render light in certain emulsions). Pure white may be in use but maybe not. Color sensibilities around safety and markings were not necessarily what we expect. And some targets are dark, most likely are in shade. OR they could be dark red or brown or even a green.
Shapes. There are the double target at Romley in a light color and the vertical light/dark target at the Tunnel on the spoil spur outside the Tunnel portal where later the turntable would be located. By at least the late 1880s target shapes seem to start to reflect use--MOW, company versus customer, etc. Or maybe just what was available to the MOW folks at the time.
And then there is that one targetless stand in later Romley in the earlier C&S era.
A lot of options for shape and color, few of which have been clearly defined. White/red is the safe path when it comes to modern viewers' expectations. I do wonder if you are going earlier--Hancock's depot did move to Romley in 1887--that this may be ok, but that light value could also be a yellow or even a blue, colors that can render as light in certain period emulsions.
Dave Eggleston
Seattle, WA