Theo,
I believe you can google pan evaporation and find the official pan design. Old data did not use the new standard. There is a color specified, but I do not remember what it was. I do re-call it being a 4' wide 1' deep straight sided pan.
Here is some information I had saved with the coeficients used in the calculations. it has some information listed on data sources you might want to look into. Hope this helps.
About Evaporation/Precipitation Data for Texas

Data Sources: Monthly pan evaporation and precipitation data obtain from the following sources: (1) TWDB and NWS Evaporation Stations; (2) Hydrosphere NCDC (National Climatic Data Center), Summary of the Day Compact Disc; (3) NCDC Climatological Data Monthly or Annual for surrounding States: Louisiana, Arkansas, Oklahoma, and New Mexico; and (4) Other internet data sources.

Monthly gross evaporation rates are computed with a GIS based program called ThEvap. This program replaced an older program, WD0300, previously run by the Texas Department of Water Resources. ThEvap runs on a Unix workstation and applies updated evaporation pan coefficients to observed daily pan evaporation measurements to calculate monthly reservoir evaporation rates for Texas.

ThEvap has been used to recalculate gross evaporation rates between 1954 and 2004. Evaporation computed from data prior to 1954 is still only available as output from WD0300. Prior to 1954, WD0300 used pan evaporation data from non-standard pans, which allowed for a much larger dataset. However, updated pan coefficients were not calculated for non-standard pans, so the decision was made to keep the WD0300 data from 1940 through 1953, but to separate it from the ThEvap dataset from 1954 to 2004 (for 1940-1953 Evaporation). TWDB also ran ThEvap to produce average monthly rainfall by quadrangle from 1941 to 2004.

The pan coefficients used in ThEvap were developed in response to the National Weather Service's NOAA Technical Report NWS 33 in 1982. They reflect both a seasonal and spatial distribution, instead of the previous annual coefficient (TWDB, Report 64, 1967). The new coefficients resulted in reduced annual evaporation rates (the new pan coefficients table) .

For the monthly gross evaporation rates, there are some missing data had been replaced by the long-term mean values of Quadrangle.

Designation of Quadrangles: the smallest practical area for evaporation calculations was a square 1 degree latitude and 1 degree longitude in size.

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Mike