FWIW, Amazon (UK) found several types of  'surface colour checkers' that use a small, hand-held sensor plus Bluetooth to app for £ 60 ~~ 100 UKP, $ 80~~140

Datacolor, Nix etc.

Expensive, but fraction of price of 'Pro' kit. YMMV...

I'd suggest that even if whatsit's kit includes a basic grey and colour-correction test-card, you get your own, plus spare, to be stored in dark, 'to be sure, to be sure'...

Think pristine 'challenge weight' for trad shop or kitchen scales, sanity-checking their battered 'working' set of brass or iron lumps. 
Likewise, for 'self-calibrating' balances, which have some very weird failure modes, such as 'whisker' growth in the load-cell creating an 'exasperation of stiction'...