For loop through servers with custom ports (for i in "user1@server1 -p 12345" "user2@server2 -p 54321" …; do)

For a for loop that can loop over more than one variable, switch to zsh. You’re already using zsh syntax by not quoting your variables:

# zsh
for colour host port (
  green user1@server1.domain1.com 12345
  blue  user2@server2.domain2.com 54321
) {
  print -rP "%F{$colour}%B$host:$port%b%f"
  ssh -p $port $host 'crontab -l'
}

To loop over lists of arguments, you could use a shell with multidimensional arrays such as ksh93:

# ksh93
ssh_args=(
  (-p 12345 user1@server1.domain1.com)
  (-p 54321 user2@server2.domain2.com)
)
for i in "${!ssh_args[@]}"}; do
  printf 'e[1;32m%se[mn' "${ssh_args[i][2]}"
  ssh "${ssh_args[i][@]}" 'crontab -l'
done

Or have the elements of the array containing strings with the arguments concatenated using a character that you know doesn’t occur in the strings and split them. zsh does have a ${(s[x])var} splitting character.

ksh and bash also do but it’s only via the clunky split+glob one that is invoked implicitly when you forget to quote an expansion there. You could use it as:

# ksh93 / bash / yash / mksh / zsh --emulate ksh
ssh_args=(
  -p:12345:user1@server1.domain1.com
  -p:54321:user2@server2.domain2.com
)
IFS=:; set -o noglob # tune your split+glob operator by specifying
                     # the separator and disabling globbing
for joined_args in "${ssh_args[@]}"; do
  args=( $joined_args ) # invoke split+glob
  printf 'e[1;32m%se[mn' "${args[2]}"
  ssh "${args[@]}" 'crontab -l'
done

That’s basically what you did in your approach except you used space instead of : to separate the elements, but didn’t set $IFS and didn’t disable glob.

The fact that your $i wasn’t split could be explained either by $IFS being set to the empty string, or the code being run by zsh instead of bash, where neither splitting nor globbing is done implicitly upon parameter expansion.

In zsh, if you want IFS-splitting, use $=var instead of $var, and if you want globbing: $~var. bash’s $var is like zsh’s $=~var. So in zsh, you’d need:

# zsh
ssh_args=(
  -p:12345:user1@server1.domain1.com
  -p:54321:user2@server2.domain2.com
)
IFS=:
for joined_args in $ssh_args; do
  args=( $=joined_args ) # split on $IFS or better:
  args=( ${(s[:])joined_args} ) # split explicitly on : without
                               # having to touch $IFS
  print -rP "%F{green}%B$host[-1]%b%f"
  ssh $args 'crontab -l'
done

A portable approach that would work in sh would be to write it:

# sh / bash / dash / ksh / yash / zsh...
while IFS=' ' read<&3 host port; do
  {
    printf '33[1;32m%se[mn' "$host:$port"
    ssh -p "$port" "$host" 'crontab -l'
  } 3<&-
done 3<< 'EOF'
user1@server1.domain1.com 12345
user2@server2.domain2.com 54321
EOF

Omitting the -r option to read allows the values to contain the separator (here space) by prefixing them with a backslash like in:

john doe@server1 1234

But contrary to the other approaches, it doesn’t allow those values to contain newline characters (likely not a problem for user, host, service names or port numbers).

Another portable one to loop over lists all of which have the same $n number of elements and that doesn’t have that limitation is to store all the elements in the positional parameters and loop over them in a while [ "$#" -ge "$n" ]; do something with "$1" "$2"...; shift "$n"; done loop. So here:

# sh / bash / dash / ksh / yash / zsh...
eval "$(
  printf "fg_%s='33[3%sm' " 
    black 0 red     1 green 2 yellow 3 
    blue  4 magenta 5 cyan  6 white  7
  printf "bg_%s='33[4%sm' " 
    black 0 red     1 green 2 yellow 3 
    blue  4 magenta 5 cyan  6 white  7
  printf "attr_%s='33[%sm' " 
    reset '' bold     1 dim     2 italics 3 underline 4 
    blink 5  standout 7 reverse 7 secure  8
)"
    
set -- 
  "$fg_green$attr_bold" user1@server1.domain1.com 12345 
  "$bg_blue$fg_white"   user2@server2.domain2.com 54321

while [ "$#" -ge 3 ]; do
  colour=$1 host=$2 port=$3
  printf '%sn' "$colour$host:$port$attr_reset"
  ssh -p "$port" "$host" 'crontab -l'
  shift 3
done