The color card matches refers to HUE not saturation..
so if it looks the same but lighter? it is the same
It is still either looks bluer or greener (or redder to more yellow) no matter if it is light, whitish, or dark (dark sure makes it easier)
Holding next to or away really should NOT change the hue, only the saturation.
Though I believe the tube is to be held ON the card
(when in doubt I always use the LaMotte kit I bought in a panic before going to an lfs