The property for the label is the color, which is
"
http://schemas.microsoft.com/mapi/id/{00062002-0000-0000-C000-000000000046}/82140003"
as a DASL property you can use in Redemption. That's a PT_LONG, so you set
it to an int value such as 1 for Important, 2 for Business, etc.
The actual text for the corresponding label colors is stored in a PT_BINARY
property on the folder, the property has a property tag of 0x36DC0102
("
http://schemas.microsoft.com/mapi/proptag/0x36DC0102" in DASL).
You'd have to reverse engineer that binary if you want custom label text.
Offhand in a MAPI viewer it looks like it's an array where a default entry
is represented by a Unicode null (0x0000) and a custom label is then stored
in Unicode in little-endian style ("F" would be stored as 0x4600.