Hyacinths bulbs think it is spring

[Lilington]

I put some hyacinth bulbs (saved from last year) in a big outdoor planter two weeks ago and they are already sprouting green tips.

I presume I was a bit too early in this mild weather.

Yesterday I put some more soil on top to tuck them in for winter but now wondering if this was the right thing?

They should go back to being dormant once the temperature drops - although the frost may damage any leaves that are outside, so I suspect you did the right thing by covering them :)