Hello and welcome to HCM,
Stab wound to the thighs is likely to cause lot of bruising.
Bruising is caused due to hematoma formation or collection of blood in soft tissues.
Hematoma changes color with passage of time.
Initially, it is blue-black in colored and slowly the color changes from blue-black to green to yellow and then finally it disappears.
Please ensure the wound is properly dressed and an antibiotic cover is required to prevent infection.
Once the wound is dressed properly, antibiotic cover is proper, there is nothing to worry about the discoloration.
It will disappear within a weeks time.

Thanks and take care
Dr Shailja Puri