Is it possible to use the multi-line output of a grep run of one file as patterns for use with a subsequent second grep run on another file?

Example:

• content of file1.txt

  2 blue
  1 red 
  1 green
  2 black
  2 orange
  

• content of file2.txt

  2 blue triangle
  2 blue circle
  3 blue triangle
  2 red triangle
  3 green circle
  4 red square
  2 orange circle
  2 brown circle
  

• Result of first grep:

  $ grep 2 file1.txt 
  
  2 blue 
  2 black
  2 orange
  

Now, I am looking for a command that implements something like

grep <PREVIOUS OUTPUT> file2.txt

that would find all lines in file2.txt that start with one of the lines produced by the grep run on file1.txt, so the desired result is:

2 blue triangle
2 blue circle
2 orange circle