A pack of four pieces of polyester photographic filter material, in the colours red, blue-green, yellow and green. For technical reasons, the colours shown in the photos are only approximate; the blue-green is a particularly poor rendition of the original.
Both blue-green and green are supplied to give the correct shade if you are lighting your signals. Use the blue-green if you are using a yellowish light source, such as a grain of rice bulb. Use the green if you are using a whitish light source such as a white LED. Depending on the railway company being modelled, either can be correct for unlit signals, with blue-green being by far the most common.
Glazing your signals is most easily carried out if the signal arm/spectacle plate unit is painted and glazed before it is finally attached to the signal post. Cut pieces of the correct colours roughly to the shape of the spectacle plate apertures, then fix them in place using gloss varnish. When thoroughly set, trim to shape using a sharp scalpel. Do not use superglue as this will fog the lens.
For anyone else out there with a knowledge of colorimetry, the exact CIE colour specifications are:
Red: x=0.699, y=0.285, Y=0.093
Blue green: x=0.113, y=0.280, Y=0.165
Yellow: x=0.496, y=0.462, Y=0.639
Green: x=0.084, y=0.795, Y=0.150
(Illuminant C, observer not stated)