Oh boy! I'm a massive fan of special effects in my polish, and this is pretty much the most fun I've had with my nails in a while... BYS Colour Change in Blue. When warm, it's a pretty jade/turquoise, but when cold, it turns a medium blue. The best thing is that it's always changing - the combinations are pretty endless. So far I've seen:
Jade nail, blue French tips (taking my warm hands out into a cold room)
Blue nail, jade French tips (blowing my freezing cold hands with a hairdryer for not-long-enough)
Gradient with jade at the base and blue at the tips (the most common - e.g. warm hands, cool draught)
Blue nails jade spots (resting my nails on a warm mug of tea on a cold night)
Jade nails blue spots (resting nails on a beer glass in a warm pub, splashing cold water on my nails)
Also, your fingers can all be completely different, and you can get a whole hand of blue and the other hand will be green!
More fun swatches:
French tip blue:
I spent a good four days looking down whenever I felt warm or cold to see if my nails had changed colour, and touching anything warm or cold I could get my hands on! :)
What's the funnest mani you've ever worn?