Bite the bullet and paint it white, NASA launched two (yeah I think its two) missions before they figured out that the paint weighed in at 600 lbs! With the cost/weight ratio on the shuttle its no wonder that they stopped.
Check out this image
http://en.wikipedia.org/wiki/Image:Space_Shuttle_Columbia_launching.jpg
FYI
Standard Weight Tank
The original ET is informally known as the Standard Weight Tank (SWT). The first two, used in STS-1 and STS-2, were painted white. As a weight-saving measure, Lockheed Martin ceased painting the external tanks beginning with STS-3, leaving only the clear primer over the now-trademark rust-colored insulation, saving approximately 272 kg/600 pounds of weight.[1]
After STS-4, several hundred pounds were eliminated by deleting the anti-geyser line. This line paralleled the oxygen feed line, providing a circulation path for liquid oxygen. This reduces accumulation of gaseous oxygen in the feed line during prelaunch tanking (loading of the LOX). After propellant loading data from ground tests and the first few space shuttle missions was assessed, the anti-geyser line was removed for subsequent missions. The total length and diameter of the ET remain unchanged. The last SWT tank, flown on STS-7, weighed approximately 35,000 kg/77,000 pounds inert.
[edit] Lightweight Tank
Beginning with the STS-6 mission, a lightweight ET (LWT), was introduced. This tank was used for the majority of the Shuttle flights, and was last used on the ill-fated STS-107 Space Shuttle Columbia flight. Although tanks vary slightly in weight, each weighed approximately 30,000 kg/66,000 pounds inert.
The weight reduction from the SWT was accomplished by eliminating portions of stringers (structural stiffeners running the length of the hydrogen tank), using fewer stiffener rings and by modifying major frames in the hydrogen tank. Also, significant portions of the tank were milled differently to reduce thickness, and the weight of the ET's aft solid rocket booster attachments were reduced by using a stronger, yet lighter and less expensive titanium alloy.
[edit] Super Lightweight Tank
The Super Lightweight Tank (SLWT) was first flown in 1998 on STS-91 and has been used since with only two exceptions (STS-99 and STS-107). The SLWT is basically the same design as the LWT except that it uses an aluminum/lithium alloy (Al 2195) for a large part of the tank structure. This alloy provides a significant reduction in tank weight (~3,175 kg/7,000 lb) over the LWT. The disadvantages of the SLWT are its increased cost (~$5 million) and production time (~4 months) when compared to the LWT. Although all ETs currently produced are of the SLWT configuration, one LWT remains in inventory and can be used if requested.