Do we have something like grayscale?
tyy for answer
No.
While blending modes were recently added to Tiled (not in the release versions yet AFAIK), none of these allow setting a layer to greyscale, or allow using a grey layer to greyscale everything beneath it. Tiled supports the hardware-supported OpenGL blending modes since those are what developers are most likely to have easy access to, but HSV adjustments are not among those.
You can use scripting to create a greyscale preview, or to even modify your tilesets to be greyscale, but these would not be cheap operations (no help from GPU) and the latter can be tricky, you’d have to be careful to return the tileset back to how it was.