I like to see how different places are making shape and or colour searching possible, like the Bayerischen Staatsbibliothek and their image based similarity search. This is really interesting to explore as you look for things like the the shape/image chosen.
Along with the many other wonderful things the Cooper Hewitt Museum is doing, there is a colour browsing/discovery for part of their collection database.
You do a search (or use one of their many discovery/browse options), and under each object which has a digitised image on the website there are up to five coloured squares, reflecting the main colours in the item. There is an option to select all colours and see difference colour schema used to describe the colours. From here you can click on any colour, and you will be taken to other items containing the same colour. This is lovely, and it is very tempting to keep trying different colours (and I quite like their Please don't steal our images, yeah? on the main image page for each item). There are lots of other ways the items are connected for browsing/discovery such as the medium where you are also prompted about similar things. This is very helpful.
Make sure you scroll to the end of the item page as there are ways which you can connect your images with the images online (through unique tags for each item).
I also like the way the content is shown here with percentages of the online collections includes in the data.
This is really a lovely catalogue/database to play with, and you can share the items on Pinterest, Tumblr or Twitter (but it would be rather fun if you could embed them with all the metadata attached - and I may have missed that this is possible amongst all the other wonderful things you can do).
There is a Random button to click which delivers (not surprisingly) a random collection item.