Follow a text stream into a destination

Fresh guided input, one value on each line:
amber
teal
amber
amber

Commands for that guided input in a fresh owned streams directory:
printf '%s\n' amber teal amber amber > practice-colors.txt
cat practice-colors.txt | grep amber
cat practice-colors.txt | grep amber > practice-selected.txt
cat practice-selected.txt

Predicted saved result: three amber lines. A second append adds three
more; replacement keeps only the current run. stdout carries ordinary
results, stdin supplies incoming data, and stderr carries errors.

The separate colors.txt source and commands below belong to the
question cards. Trace them independently without copying the activity
result. They use different input and a different matching word.

Worked: printf '%s\n' red blue | grep blue
Predicted stdout: one blue line.
Separate question source: red, blue, green, blue on separate lines.
cat colors.txt | grep blue > selected.txt
Use the separate colors.txt lines to predict selected.txt yourself.
> replaces destination; >> appends; | connects stdout to next stdin.
stderr is a separate error stream unless explicitly redirected.
Question: printf '%s\n' gold blue | grep blue
Which command receives printf's stdout? Enter the command name only.