It called rit dipping.
Its special "paint" for clothing.
You can buy it in powder and in liquid form, doesnt matter both are good.
Just boil some water, trow in the rit, stirring with a kitchen spoon, let it cool down around 70 degrees Celsius.
And then hang the backcover, ( with a thin piece of wire through the hole ) in the hot water.
Repeat this action in a couple of steps and under continues survailance.
Tips
Is the water to hot, it will disform your cover, is it to cold the diy wil not penetrate enough.
You can only work from light to dark.
So a white one should be perfect for the job
Its a one way ticket, no return
And be careful with your clothes and skin.
Make good preperations, the object should be clean and free of any fingerprints.
No scratches ( you see every scratch because of the structure of the damaged plastic ).
Wash the residu as soon as possible from the object ( stains )
Use something else to try it with til you get the hang of it.
We used that same trick for RC cars and railroadmodeling.