I got the idea on the croatian forum, where some girls put holo polishes on top of other polishes and got awesome holo effect. I tried it with Purple Diamond since it's the most visible holo polish I own, and it doesn't have strong color on it's own, that could 'kill' the base polish.
As a base I used Essence Chuck, one of the best Essence's polish. It's opaque in one smooth layer and dries in no-time.
I added just one coat of Purple Diamond and here's the result:
Unfortunatelly, these two last days there was no sunshine at all, so I got to enjoy the holo effect only under the artificial light.
Next time I'm watching the weather forecast before I put on a holo polish!