#!/bin/bash
s='GACCTAAATTTGGGCCCAGGTC'
 
# Original script
myVar=$(grep --color 'GACCT[ATCG]*AGGTC' <<< "$s" | cat)
myVar1=$(echo ${myVar##*GACCT})
echo ${myVar1%%AGGTC*}

# My suggestion:
grep --color=never -oP 'GACCT\K[ATCG]+(?=AGGTC)' <<< "$s"