Turn on one light in a series like a christmas tree light string
You can use strings of WS2811 digitally-addressable RGB LEDs.

Each WS2811 provides RGB colour from 3 x 8-bit digital input data.

Each contains 4 connections: serial input, serial output, ground and 5V. The devices can be connected in long daisy chains, with one WS2811's serial output driving the serial input of the next WS2811 and all 5Vs and GNDs in parallel. You can buy them pre-assembled into chains of ten for a fiver, longer chains for more.

Have a look on the interweb for WS2811, the datasheets and supplies are freely available. I think there's an Arduino driver for them so there may well be Raspberry Pi software, too.