How to write code in bash?

Question

bash

set SUB 'blue'
set rgb [open rgb1.txt r]
while {[eof $rgb] == 0} {
  set colorstr [gets $rgb]
  if {$colorstr != ""} {
    if {$colorstr == ??????} {
      .lb1 insert 0 $colorstr
    }
  }
}

There is a script fragment that reads a line from files and adds it to the listbox
Task: Find the substring 'blue' in the string $colorstr and if found, then perform the action
What should I write instead of ?????? ?