The Adafruit Neopixel library is Esplora compatible, having confirmed this last night.
I used a 4x4 matrix and ran it all with an external 5V supply.
(The Neopixel ‘commands’ have to be used for this - and not the Esplora.write LED stuff - just to make that clear.)
An external supply isn’t absolutely necessary. If you have a few neopixels or similar, it’s OK to use V_usb, but if you get a 4x4 or something and try and make them all ‘white’ then that much may over-burden “Mr. USB”
Adding wiring diagram. NB - All matrixes may not have the same pin-out as mine. The resistor is NOT optional. I used a 0.1uF decoupling cap (not shown) at the matrix power pins; good practice but maybe not essential.