There are subcutaneous, intradermal, intramuscular and intravenous injections. Subcutaneous and intradermal injections usually do not cause bruising. The bruising and purple after intramuscular injection is bleeding caused by piercing the capillaries during the injection. If blood enters the syringe, the injection site needs to be changed.
A bruise from an intravenous injection is considered to be localised blood seepage caused by damage to the wall of the punctured vessel and needs to be pulled out and pressed for a while before reinjection.