IIRC, Windows 2000 had an overall bluer theme that reduced the green background and orange-tinged 3D colors of 98, and brought in a new set of icons. Also, it added alpha blending to GDI and started using it in Explorer.
It also used a lighter base gray than 95/98 did, which contributed to it feeling significantly less dreary. This is why for me, the canonical “classic Windows” theme is the 2k version.
The base color also had an off-white beige tone to it. To this day I'm curious if this was made as an attempt to counter balance the usual white balance of pc monitors at the time, which where set at or above 9300K, producing a blue-ish tint.
When win2000 was still in testing (was this called Memphis, can't remember) the UI elements like the X to close the window would light up in blue. Never made it to release.
