Best Loudspeakers for Rich Timbre?

Though I consider them more accurate than Technicolor, this term you used is exactly how the Soundstage review of the Focal Sopra 2 described them.
https://www.soundstagehifi.com/index.php/equipment-reviews/890-focal-sopra-no2-loudspeakers

All the bbc design speakers have it in spades.
Amphion speakers have it too, although they have a different sound.

+1 @david_ten

Correct. The speaker should be as transparent and neutral as possible. It is not possible to remove resonance or euphonic syrupy timbre once you have it. However, starting with a speaker that is clean and neutral at all dynamic levels across the entire frequency range and you can then begin tailor the sound with your favourite SS or tubes.

My recommendation is neutral and clean, accurate speakers with an excellent highly accurate powerful SS power amp and then add coloration to taste using TT cartridge, phono pre, preamp and or DAC.

For me the OP has it all backwards. I agree with David 100%.

I have no idea what is a "neutral" sounding speaker.  That implies some kind of reference; what is that reference?  The correct speaker is the one that sounds most musical to the beholder.  It would help if the original poster would provide a personal reference of stuff that sounded good and stuff that sounded to thin or bleached of "color."  At best, I can hazard a guess that this person is looking for something from the likes of Audio Note, the O93 or O96 DeVore, Vandersteen, Harbeth (particularly the 40.2), ProAc, JM Reynaud, Trenner and Friedl, and Charney.  There are some speakers that may sound a bit bright on top that still manage to deliver weight, upper bass warms, and natural timbre (e.g., Triangle speakers).

The very best speakers, in terms of delivering saturated harmonics and timbre, to me, are horn-based systems, but these require some expertise to assemble or are quite pricey; they involve finding and using older/exotic drivers or really expensive new drivers (e.g., G.I.P. Laboratories).

If you are willing to put in some effort to hear stuff that is quite unique, check out the custom-built-out-of-vintage parts speakers at Deja Vu Audio in McLean Virginia, or at the satellite stores (Deja Vu South-Southern Florida, or Deja Vu West-La Jolla, Ca.).