Challenges are about challenging artists to work with limited tools or otherwise outside of their comfort zone.
What is the challenge here? There are so many diverse colours available here that an artist could do just about anything without really thinking about it, without the challenge restrictions affecting their decisions in any meaningful way. This isn't a challenge, this is a minor inconvenience - "oh no I can't use the colour sliders."
There's a reason that palette challenges tend to employ tiny palettes. Those really force the artist to think about the colours they're using, to use and mix them in interesting way. If you're providing full ramps of just about every hue, even if the total number of colours is much smaller than the entire RGB space, you're not really limiting anyone.