Looking around through several available styles in order to find what works best for me, I noticed that the definition for 0x08 (minor ramp) is skipped, so these ramps would be displayed whatever the default is on the given device.
The default mkgmap style uses this both for primary and secondary links. I would suggest to set it to the color of secondary highways, as in my opinion a primary road with a “lower class” ramp/link looks more plausible than a secondary with a “higher class” ramp/link.