Wait...shades? How can one quantify the decrease (and how does it handle "overflow", i.e. at what warp value is "maximum purpleness" attained, and what happens if you exceed that value)?
Temporary warp is gained by things like eating brains and getting attacked by things in Thaumcraft's Outer Lands dimension. It gradually decreases on its own, and is nullified entirely by Sanitizing Soap. The Sanity Checker represents it as a light purple bar atop of the darker bars for the other types of warp.
"Sticky" warp (or just Warp, as it's called in-game) is gained by researching forbidden knowledge and from crafting items gained by forbidden research. It does not decrease on its own, but can be removed a very small amount at a time by the application of Sanitizing Soap. You have a better chance of removing a bit of this type of warp if you use the soap while under a Warp Ward buff. The Sanity Checker represents this as a medium purple bar between the other two bars.
Permanent warp can only be obtained by the more warping forbidden researches. It cannot be removed by any means available in vanilla Thaumcraft, and the Sanity Checker represents it as a dark purple bar at the bottom of its meter.
As for what happens if your total Warp level would exceed the top of the Sanity Checker... I don't really know. I would guess it's capped.
It's also worth noting that it's impossible to fill the Sanity Checker more than a fifth of the way or so with permanent warp without a bunch of mods adding forbidden knowledge; and sticky warp isn't easy to get a lot of either. However, in the Outer Lands, it's not hard to fill it to the top with temporary warp, especially if you're not careful around those glyph stones (which zap you, causing a little damage and some temporary warp).