The stores are full of cheap Halloween decorations that light up, but what use are they if you can't do something with them!
Most of the light up decorations (that I've come across) are a outside casing (in my case a ghost), and a small plastic block in the bottom.
This electronics module is 3 coin cell batteries connected to a colour cycling LED along with a switch to break or make the circuit.
The easiest modification to make to something which changes colour, is to make it controllable.
I always keep Adafruit Diffused Through-Hole Neopixels on hand for this purpose.
https://cpc.farnell.com/adafruit/1734/neopixel-8mm-thru-hole-diffuse/dp/MK00281?st=neopixel
First step is to desolder the original colour cycling LED
The next step is to solder the +5V pin of the Neopixel to the switch output and the GND pin to the - battery terminal.
For the control of the neopixel I add a wire to the Data in pin, and connect an extra wire to the GND pin.
This is because the ground of the neopixel needs to be shared with the microcontroller for the comms to be understood, however the neopixel can be powered from the coincells which avoids a 3rd wire.
(The AG13 batteries will produce 4.5V (1.5V*3), but this is fine for these neopixels, they will just be dimmer).
I then feed the wires through a hole if available, or make a small hold in the case.
Now for controlling the decoration you can pick your microcontroller or pi of choice.
In the past I have typically have done this modification to take into a local school where I run a codeclub.
During these sessions we control our ghosts with a Micro:Bit.
I have example code available here:
https://github.com/hifromkatie/microbit-halloween
microbit-neopixel is written in MakeCode and changes the colour depending on which button is pressed or how it is held.
Adj-colour-tilt is written in Python; Button A increases value, Button B decreases value. If tilted left, buttons will adjust R(red) value If level, buttons will adjust G(green) value If tilted right, buttons will adjust B(blue) value
However these are just demos/examples to show the possibilities, so experiment and have fun creating your own programs and light displays 
MakeCode code:
Python code:
(A different Ghost, same mod)
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