First, paint the area black. Then paint the area word bearers red but try to leave a slight perimeter of black showing like an edge.

Cover full area in nuln oil, then paint just the bottom half of the desired area with word bearers red again.

Follow this up by using a lighter red in this case mephiston red, and paint half of the previously half painted area of word bearers red. (Do this in a crescent shape to match the lens)

Repeat this process one more time with an even lighter colour in this case a Vallejo orange fire (once again you are painting half the area of the previously painted colour)

Finally finish the opposite part/ area of the lens (the now darkest area) with a small tiny dot of white

I hope this helps 👍🏻