This is a Mother's Day Project I just finished up for Mom.
It's based on a Mega328 Arduino running at 1MHz with a custom bootloader. It draws just 0.08mA in standby mode and when activated by motion, cycles slowly through the color spectrum. It takes about 10 triggers to make it through the rainbow.
It has a CDS sensor so it only lights at night time.
The PIR sensor is amplified with a micropower TLC1079 quad opamp.
Battery power is from 3xAA and it draws about 50mA when lit...
The enclosure is CNC milled from applewood and assembled with finger joints.
The blog post contains the schematic, the code, bootloader, video and all the details.
Happy Mothers Day!