cat << EOF | sed -e "s/^[^:]*/x1b[31m&x1b[0m/g" | sed -e "s/to/x1b[31m&x1b[0m/g"
foo: 1 to 2
bar: 3 to 4
quux: 5 to 6
EOF

s/regexp/replacement/
       Attempt to match regexp against the pattern space.  If  success‐
       ful,   replace  that  portion  matched  with  replacement.   The
       replacement may contain the special character & to refer to that
       portion  of  the  pattern  space  which matched, and the special
       escapes 1 through 9 to refer  to  the  corresponding  matching
       sub-expressions in the regexp.