▁post mark ▁could ▁use ▁black ▁in k . ▁ ▁The ▁majority ▁of ▁post marks ▁today ▁are ▁in ▁black , ▁with ▁red ▁( part icular ly ▁in ▁the ▁United ▁States ▁with ▁local ▁post ▁offices ' ▁hand st amps ) ▁following , ▁though ▁sometimes ▁they ▁are ▁in ▁other ▁colours . ▁ ▁This ▁is ▁particularly ▁true ▁in ▁the ▁case ▁of ▁p ict orial ▁post marks ▁if ▁the ▁colour ▁in ▁question ▁has ▁some ▁connection ▁to ▁the ▁comm em oration . ▁ ▁Digital ▁post marks ▁ ▁In ▁ 2 0 0 4 ▁the ▁United ▁States ▁Post al ▁Service ▁announced ▁plans ▁to ▁introduce ▁first ▁day ▁digital ▁colour ▁post marks ▁to ▁be ▁used ▁to ▁cancel ▁some ▁first ▁day ▁covers ▁for ▁comm emor ative ▁st amps ▁in ▁ 2 0 0 5 ▁and ▁this ▁practice ▁has ▁continued ▁into ▁ 2 0 0 6 ▁and ▁was ▁on going ▁as ▁of ▁ 2 0 1 5 . ▁ ▁Post mark ▁advert is ement ▁ ▁Singapore ▁Post ▁offers ▁a ▁" post mark ▁advert ising " ▁service ▁which , ▁strictly ▁speaking , ▁applies ▁to ▁the ▁" k iller " ▁rather ▁than ▁the ▁post mark . ▁ ▁Hung arian ▁Post ▁Co ., ▁Ltd . ▁offers ▁a ▁similar ▁service . ▁ ▁S tere os cop ic ▁ ▁There ▁have ▁apparently ▁been ▁some ▁post marks ▁with ▁a ▁producing ▁a ▁s tere os cop ic ▁or ▁" 3 D " ▁effect ▁where ▁a ▁special ▁vie wer ▁is ▁required ▁and ▁is ▁considered ▁more ▁as ▁a ▁novel ty ▁than ▁a ▁practical ▁post mark . ▁ ▁Val u ation ▁of ▁can cell