Adjust grayscale color ramp

[trvrb]

The existing grayscale color ramp (used for values absent in an explicitly specified color scale) had values that were too dark and threw off the overall color balance. This commit narrows the grayscale color ramp to be more in line with pastel color ramp.

I mainly did this because I didn't like the aesthetics of the new clades coloring. Here it is on current Auspice master:

And here is this PR:

[trvrb]

I've updated this PR to inject a bit of color into the "grayscale" color ramp. I'm done this for both extra values not explicitly defined in the categorical color ramp, and also for the "unassigned" values (?, etc...). I'm rather pleased with the results, which seem to place these "null" colors more in the same universe as the default pastel color ramp.

@jameshadfield: This is ready to merge from my perspective.

[jameshadfield]

Happy for these changes to go through ✅

In the future we may want to better distinguish between "extra values" (i.e. defined traits which weren't assigned a colour by the dataset) and "unknown values":

P.S. It's not this simple, as auspice doesn't render hex values truthfully for the tree. We make branch colours closer to grey, brighten terminal nodes and brighten legend swatches by a different (!) amount. I believe colours in the frequencies panel & the map are rendered truthfully, but I may be wrong. Here's a side-by-side of current auspice (where we lighten things) and a version where we don't:

[trvrb]

Distinguishing between Auspice assigned colors for traits that don't have an explicit listing and traits that are "unassigned" is a good argument to keep unknownColor at #AAAAAA.

We could have a situation that involves both kinds of grays and I can see it being useful to distinguish.

I didn't think about all the lightening we've been doing. I'd probably aim for center of tip bubble to be the exact color that's specified and have the center of the legend swatch also be exact color specified. We then lighten the edge of the tip bubble and the edge of legend swatch by the same amount.