If you have access to the

If you have access to the printer's resource Pantone Matching System color books, you could get a PMS color value for these items by visual estimation.  Hold up the book to a known color sample, and find the best match.

Some graphics programs such as Corel allow you to pick a PMS color, then change to RGB, but be aware that the conversion isn't perfect.

Its "iffy" business to try to match a color sample to a monitor screen color, as the various screens have different temperature values, and so you can get some undesirable color shifts to yellow or blue.

Found Matches, I hope

I found a reference that matched the colors to Floquil paint mixes, and a reference that gave the RGB values for those paints.  The final results for the 1920s thru 50s. I assumed that in mixing 2 colors the RGB values would fall between the 2 base color values (ie. Red values of 110 and 150 would be a final red value of 130). The floquil replaced the Polly Scale paints.

Siding SF Colonial Yellow RGB  229,218, 185 equal parts Floquil UP Armor Yellow & Reefer White 

Trim    SF Bronze Green   RGB  83,101,77      equal parts Floquil Southern Sylvan Green & Pullman Green

Roof   Floquil Mineral Red RGB  162,47,44   This I had no RGB reference for, but I found a color swatch on the Floquil site, and used a graphics program to analyze and then compare the results.

The SF Paint reference site is:

http://www.atsfrr.com/resources/KJordan/SFRHMS%20Painting%20Guide.pdf

The paint RGB reference is:

http://www.art-paints.com/Paints/Enamel/Floquil/Floquil-Railroad.html

If I made a mistake in my assumption about how to determine the RGB for mixed paints, I would appreciate advice on how to do it correctly.