You will need three colors of paint for the tables, and additional paint colors for the mountain. Remember, Lionel used oil based paints back in the 1950s, so there is nothing exactly like the originals available today. The following formulas are for Sherwin Williams custom mixed latex paints that were matched to several original Lionel layouts. However, the colors did vary somewhat from layout to layout, so there is no definitive exact color match, although the following should be very close.
Tan (for roads and track beds as well as highlights on the mountain)
BAC Col. 02 32 64 128
B1 Black - 9 - -
R2 Maroon - 6 - -
Y3 Deep Gold - 43 - -
W1 White 2 32 - -
Base: Flat extra white, 1 gallon
Green (for grassy areas on both the board and mountain)
BAC Col. 2 32 64 128
L1 Blue - 59 - -
R1 Magenta - 19 - -
Y1 Yellow 6 41 - -
Y3 Deep Gold 2 7 - -
Base: Flat deep base, 1 gallon
You need to add an extender to the green latex paint so that it doesn't dry too quickly. This allows you to sprinkle the Lionel 919 grass over it and have good adhesion and allows you to have ample working time (remember Lionel used slow drying oil based paints). I used Coronado Latex Retarding Solvent # 80-0.
Side Trim (fascia boards) (correct for displays made from 1955 onward)
BAC Col. 2 32 64 128
B1 Black - 3 1 1
G2 New Green - 6 1 -
Y1 Yellow - 4 - -
Base: Interior semi-gloss super white, 1 quart
For the mountain, Lionel used casein paints. These are available as theater paints from Rosebrand and others. To color Lionel display layout mountains from 1955 onward, use the classic earth tones: Raw umber, burnt umber, burnt sienna, and yellow ochre. Mix these freely and do not cover the entire surface of the grey felt. The paints should be "dabbed on". I like to use a 1" round sash brush for this. Also use the tan (from above) to "pick out" the edges of rock formations etc. The green paint is used for the base of grassy areas. Lionel 919 grass is sprinkled onto the green paint when it is wet just like on the flat areas of the layout. Use your imagination! No two layouts had mountains that were alike. For reference, run a search for "D165 Display Layout" and you will see photos of my display layout recreation. For the tunnel portals, the two "front" facing portals (upper and lower level) are No. 920-2 as is the "rear" facing upper level portal. The lower level portal on the lower level is cut from 3/4" pine or plywood. It should be painted grey to match the 920-2 portals. There is also a small access portal at the rear of the mountain so that derailments etc. can be cleared. This portal should be painted in the same tan as the roads and track beds. BTW, once the track along the right side of the layout enters the tunnel, it is supported by a narrow plywood platform that ends at the rear tunnel portal. The trestle set is not continuous through the mountain.