'dark' and 'light' colours are identical, but with a different font weight

Describe the bug
When setting colours for calendars, dark and light versions of 16-bit colours are identical, but the light version produces a bolded font. For example, setting a calendar's colour to brown produces the same colour as yellow, but with the font bolded. The same is true of other dark-light pairs -- dark red + light red, etc. The colours work just fine in other programs so I can only assume this is some sort of problem with khal.

To Reproduce
Ehhhh, I dunno, use khal in Kitty maybe?

Expected behavior
I would've expected yellow and brown to be different colours (the ones I have set in my Kitty conf, that is #fa9a00 and #804800 respectively)

Screenshots
If applicable, add screenshots to help explain your problem.

Here's a calendar with its colour set to brown:

And another that has it set to yellow:

OS, version, khal version and how you installed it:

• The output of khal --version: khal, version 0.13.0
• Installation method: from Arch's extra repo
• python version: 3.13.7
• OS: Arch Linux
• Your khal config file: https://gist.github.com/ngoomie/306f805d4c7389369e41e7bcc3f97ada#file-config
• The versions of your other python packages: https://gist.github.com/ngoomie/306f805d4c7389369e41e7bcc3f97ada#file-pip-freeze

Additional context
I'm using the Kitty terminal (v0.43.1)

Start by reproducing the colour settings in khal 0.13.0 under Kitty, using the linked configuration and comparing the dark/light pairs described in the report. Trace khal’s calendar-colour rendering path; done means 16-bit colour pairs display distinct configured colours without the light variant changing font weight, with regression coverage added if the project’s existing tests support it.