There are 4 types of tiny icons which occasionally appear on the goods in the City Market's "Buy Items" section. The icons show you if the collectible/material is currently required for something in your city:

1. Green house icon - the collectible/material is required for upgrading a residential building.

2. Orange plane icon - the collectible is required for completing an Airport order.

3. Magenta ship icon - the collectible is required for completing a City Port order.

4. Purple camera icon - the collectible is required for upgrading a Tourist Resort.