Here are a few basic issues to consider when recording glass colors:
    1.  Diaphaneity - The transparency of glass, also related to the
light used to view glass color, either reflected or transmitted light.
            a.  Transparent -  in theory you could distinguish printed
text through the glass.
            b.  Translucent - light is transmitted through the glass as
a glow.
            c.  Opaque - no light is transmitted through the glass.
    2.  Color Layers - presence or absence of different layers.
            a.  Monochrome - only one layer of glass color to measure.
            b.  Polychrome - two or more layers of glass color to
measure.
    3.  Color Attributes - The best classification of color attributes
has been defined for the Munsell color notation system.  A good
explanation of these attributes can be found on the Munsell web page at:
 
    http://www.munsell.com/munsell1.htm
As an example of the Munsell system, consider the Munsell color notation
of 7.5BG 5/8.  It conveys three color attributes:
            a.  Color Hue - the color of an object, e.g., on the Munsell
color wheel, 7.5BG is shade of bluish-green tending more toward blue
than yellow.
            b.  Color Value - the relative lightness or darkness or a
color, e.g., on a scale of 1-10 with 1 being very dark and 10 very
light, 5/ is medium, neither light nor dark.
            c.  Color Chroma -  the intensity of color, e.g., on a scale
of 1-16 with 1 being very dull and 16 very color rich,  8 is a
relatively strong color, neither very dull nor very bright, brilliant,
or color rich.
    4.  Color Modifiers - During manufacture, materials are occasionally
introduced into the glass to modify appearance of color (e.g., metallic
or iridescent materials).  These modifiers alter the color of the glass
for the viewer, and should be considered and described when observed.
    5.  Surface Appearance - Often the surface of the glass has been
modified either during manufacture or use (c-transforms), or by natural
erosive processes (n-transforms).  Thus, the surface appearance may be
shiny, dull, corroded, matte, pitted, iridescent (e.g., glass disease),
solarized, etc.; and this appearance alters the appearance of color for
the viewer.  For example, a matte surface appears to have a duller color
(i.e., lower color chroma) than a shiny glass surface.  When possible,
such appearances should be noted.
    6.  Glass Thickness - Glass of exactly the same color but with a
different thickness may appear to the viewer as different "colors."
Inexperienced individuals may confuse color value with color hue when
describing or measuring colors.  This is a difficult issue to resolve,
especially for transparent and translucent glass.  With opaque glass,
one is always measuring color as reflected light, while with transparent
or translucent glass, one may be measuring color as reflected or
transmitted light.  If measuring transmitted light, the thicker the
glass the richer and darker the color (i.e., stronger chroma and darker
value).
    7.  Light Source - When observing color and measuring it with color
chips (e.g., Munsell or Pantone chips), the light source must remain
constant.  The same color viewed in daylight, incandescent, or
fluorescent light will appear different.  Perhaps the easiest light
source to duplicate in the field or a lab is a single 100-watt
incandescent light bulb about 1-2 feet from the object and color chips.
IF you rely on sunlight, there is always the problem of brightness on a
sunny or cloudy day.  It is often a good idea to use a magnifier when
viewing the object and color chips.  A jewelers magnifying visor works
best, as magnifying lamps often employ fluorescent lights and may alter
the appearance of color as viewed with the 100-watt light bulb, and a
hand-held magnifying lens is difficult to hold while also holding an
object and color chips.
 
For research and archival recordation it is probably a good idea to
employ the Munsell system (it has been the industry standard for
decades).  However, the full book of color (glossy collection) is about
$500 or higher.  Other systems can be used (e.g., the Pantone system),
but since these other vendor systems often change through time, it would
be best to convert vendor color notations into Munsell color notations.
Munsell has a free beta version of new software that allows the
conversion of other vendor color notations to Munsell notations.  If you
want to download this software, go to the Munsell web page at:
    http://www.munsell.com/cmc/index.htm
 
Hope this discussion is of assistance.  LAR