I've done some experiments in this direction and my conclusion is that _RGB returns a value depending on the color palette used in the 256 color screen. This is not necessary for a 32-bit screen and therefore _RGB returns the same value as _RGB32 in a 32-bit screen.
CONST sed~& = _RGBA32(155, 12, 127, 255)
sed2~& = _RGB(155, 12, 127)
PRINT sed~&
PRINT sed2~&
SLEEP
SCREEN 13
sed2~& = _RGB(155, 12, 127)
PRINT sed~&
PRINT sed2~&
SLEEP
FOR f = 0 TO 255
_PALETTECOLOR f, RND * 255
NEXT
sed2~& = _RGB(155, 12, 127)
PRINT sed~&
PRINT sed2~&
SLEEP
SCREEN _NEWIMAGE(1024, 768, 32)
sed2~& = _RGB(155, 12, 127)
PRINT sed~&
PRINT sed2~&