I have bought an ESP8266, WB2812 LED strip 2 metres, a bit of soldering, arduino code and a bit of integration with a Raspberry Pi. Exclude the Pi (that I have anyway) and all in for £15. I can change the colour, mode brightness etc all from Siri.
There are other options out there, but I think this is the cheapest at the moment.
An alternative is to buy an LED strip that is 'Alexa' compatible and again trying to use something like homebridge on a raspberry pi or Linux machine to integrate.