You can use the Reference colour pots, accessible anywhere in the Keyer, to sample colours. These do not affect any parameter in the key, but are convenient to store colours as you work.
For example, use colour pickers from the Reference colour pots to pick flesh tones from one clip. Then in another clip, use the colour pickers from a Sample colour pot in the Master Keyer to pick the stored colours from the Reference colour pots to key out an actor from a clip.
To store colours for reference: