By what I know the DC bios checks for some values on it's internal flash memory. The logo swirl color is hard wired to the
50/60hz ID bit on the flash. If it's 50Hz the swirl will be blue, else it will be orange. Brazilian Dreamcast outputs PAL video (60Hz PAL-M video) so it's swirl is orange.
Some modchips works by patching the DC ROM somehow it turns a PAL machine into NTSC (USA/Canada area) so you will get a orange swirl and a U/C DC that play all GD discs.
Just a comment about the brazilian DCs ... Some DC programmers think that 60Hz PAL systems do not exist. So they read the color system ID byte on the DC flashrom instead of reading the 50/60hz first, making all patched games with auto detect TV system play as PAL-B/G/I in brazilian Dreamcasts.