In a box mix together the white cake mix, oil and eggs until combined. Divide the batter equally into two separate bowls.
Add gel food colouring to get the desired colour you want (I used green and red gel food colouring - also see notes).
Place in the fridge for about 45 minutes.
Meanwhile, using your edible marker pen, draw ‘eyes’ on the skittles or m&m’s. I used different colours for fun. Set aside.
Once ready to bake the cookies, line a baking tray with a silicone mat or baking parchment.
Scoop out the cookie dough and bake at 350 F / 175 C for 10-12 minutes. The cookies are ready to be taken out of the oven when the edges are slightly firm but the middle is still soft and squishy (also see notes).
As soon as you take them out of the oven, press in the ‘eyes’.
Once the cookies have cooled you can draw scary or silly faces on the cookies with the edible marker pen.
These cookies can be stored unto 4-5 days in an airtight container.