I've added the 2-6 colours (the last room of this level) as well as two unused colours, the first one which use the palette from level 4-7 and the last one from levels 4-1, 4-2, 4-5 and 4-6 (which is fitting given the world is supposed to be mountaineous with one level dedicated to these platforms, even moreso with 4-7 whose main gimmick is platforming). There exist other palettes which are compatible with them but are closer to the 2-6 and 4-3 colours which means they aren't really made for Countdown Platforms as they lack the highly specific blue and green for the 3 and 4.

Yah I assumed the number itself was a sprite over the foreground tiles, though given the amount that makes sense.

That's because it's impossible for the number to be superimposed: The platforms are blocks and not sprites and you can only draw one unique tile on a given layer (conversely, that's why so many can exist on-screen at a time: They're part of the level geometry and thus don't fill in any sprite slots).

Strictly speaking, a pressed platform is a sprite but its only purpose is to handle the interaction, given how the solid part of the block is off the grid in that state while the graphics themselves are still part of the foreground tilemap.

Surprised the number isn't superimposed.

Just as an aside, I looked into the ROM with Golden Egg and it turns out there are more colours out there which are valid for the platforms, though they tend to use the same shade as 6-7. That being said, there is one palette with a unique shading which could have been added to the sheet, though I don't know if they're used in the game or not.

Either way, the colours are now fixed (and apparently, 3 is used in 4-3) with the palettes as a proof.

As far as I can see, the number 3 in Stage 4-3 has a different colour:
https://www.mariouniverse.com/maps-snes-yi/

Hm... someone has updated the numbers to be more colourful. That one isn't wrong but as mentioned in the 6-7 level rip, numbers 3 and 4 actually are palette dependent and can come in other colours as well (I used 4-3 as a baseline).

Edit: In hindsight... 4-3 doesn't use higher numbers so the revision is actually justified. In fact, the two colours of the old rip could be used for other things while the palette of 6-7 specifically includes a strong blue and green which only makes sense for the numbered platforms.

Edit 2: HOWEVER!, it also affects the shade colour. So this correction is only somewhat correct, the platforms should have a green and not a blue shade as a result.