ppmcolormask: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/ppmcolormask - from package: netpbm-progs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ppmcolormask - from package: netpbm

You may install the required package with command apt-get install {package}

Error in other languages:
ppmcolormask: bevel nie gevind nie
ppmcolormask: comandă negăsită
ppmcolormask: comando não encontrado
ppmcolormask: commande introuvable
ppmcolormask: command not found
ppmcolormask: käsku ei ole
ppmcolormask: không tìm thấy lệnh
ppmcolormask: komanda nerasta
ppmcolormask: Kommando nicht gefunden.
ppmcolormask: kommandot finns inte
ppmcolormask: komut yok
ppmcolormask: nie znaleziono polecenia
ppmcolormask: níor aimsíodh an t-ordú
ppmcolormask: no se encontró la orden
ppmcolormask: no s'ha trobat l'ordre
ppmcolormask: opdracht niet gevonden
ppmcolormask: parancs nem található
ppmcolormask: perintah tidak ditemukan
ppmcolormask: príkaz nenájdený
ppmcolormask: příkaz nenalezen
ppmcolormask: команда не найдена
ppmcolormask: командата не е открита
ppmcolormask: 命令找不到
ppmcolormask: コマンドが見つかりません