cmd/perfdash: use less alarming colors
cmd/perfdash: use less alarming colors

We decided red was a bit too alarming of a color to use
for ordinary data on a performance dashboard. This
commit also makes older lines fade out more aggressively
so they're easier to distinguish.

The newest bucket (containing incomplete data) is now
magenta instead of gray.

Closes [unavailable]

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain