At the time of designing the boards it felt like a no brainer to use a BlinkM clone over I2C, because I had worked with them in my early beginnings. It took me a while though to finally burn the firmware and set the fuses right, but now I finally made it!
avrdude -c usbasp -p t45 -U lfuse:w:0xe2:m -U hfuse:w:0xdf:m avrdude -c usbasp -p t45 -U flash:w:cyz_rgb_slave_attiny45.hex
I also found out that avrdude is able to slow down the clock frequency of SPI by itself. With -B 100 I was able to reprogram the tiny45, no matter how slow I was setting the fuses by accident.
My #a "true" in system programmer helped a lot burning the firmware onto the attiny!
Of course... of course. R and G are mixed up. gnargh. But here's a demo: