I’ve updated my script and removed the pot from my design, so that now it just fades between random colours on its own, using an LDR to tell if its dark enough to turn on or so light that it should turn itself off as you wouldn’t see the LED anyway:

// init vars
const int REDPIN = 1; // 85 leg 6
const int GREENPIN = 0; // 85 leg 5
const int BLUEPIN = 4; // 85 leg 3
const int LDR = 3; // 85 leg 2
const int THRESHOLD = 200;
int last_time;
int red;
int green;
int blue;
int current_red;
int current_green;
int current_blue;

void doFade(int pin, int &val, int &current_val)
    // val and current_val are passed as references
    // so modifying them will actually modify e.g.
    // red and current_red without having to return them
    if (val > current_val)
        analogWrite(pin, current_val);
    else if (val < current_val)
        analogWrite(pin, current_val);
        analogWrite(pin, current_val);
        val = random(255);

void setup()
    // read from unconnected pin for entropy

    // setup pin directions
    pinMode(REDPIN, OUTPUT);
    pinMode(GREENPIN, OUTPUT);
    pinMode(BLUEPIN, OUTPUT);
    pinMode(LDR, INPUT);

void loop()
    // fetch current uptime
    int this_time = millis();

    // is ldr reading higher (brighter) than our threshold?
    if (analogRead(LDR) > THRESHOLD)
        // is it more than 5secs since it was last dark?
        if (this_time - last_time > 5000)
            // turn off the led as its not dark
            analogWrite(REDPIN, 0);
            analogWrite(GREENPIN, 0);
            analogWrite(BLUEPIN, 0);
        // call fade function for each colour
        doFade(REDPIN, red, current_red);
        doFade(GREENPIN, green, current_green);
        doFade(BLUEPIN, blue, current_blue);
        // reset the counter
        last_time = this_time;

I’ve soldered the final design onto veroboard after prototyping on breadboard:


I’ve contributed a few patches to arduino-mk and now you can compile for the ATtiny85 using a Makefile, however it doesn’t support the arduino-tiny core or tiny-core2 only the attiny (HLT) one, so I couldn’t use it for this as I needed the 3 PWM pins which is not supported by attiny.

Update: I seem to have made it work:

ARDMK_DIR          = /usr/share/arduino
ARDMK_PATH         = /usr/bin
AVR_TOOLS_DIR      = /usr
ISP_PORT           = /dev/ttyACM0
BOARD_TAG          = attiny85at8
ALTERNATE_CORE     = arduino-tiny
ARDUINO_VAR_PATH   = /home/user/arduino/hardware/arduino-tiny/cores/tiny
ARDUINO_CORE_PATH  = /home/user/arduino/hardware/arduino-tiny/cores/tiny

include /usr/share/arduino/Arduino.mk