Black backgrounf for the object that colourized and they are transparent

Hello! Can you please check that you don't have a black background set for the whole page. Go to the Background colour and change it to No Fill if it does have black as the colour. Then when you export (either PNG or PDF) it should remain transparent. 

If that isn't the cause, can you explain a little more please? Which part of the image is the vector that you colourised? And how did you colourise it?