wallpaper paste mixed with paint makes good colourful slime do it in the bath or shower the slop will just wash away.
best mix is 5litres of water to one sachet of paste
before mixing it add food dye or washable paint to the water as its hard to colour once its mixed.
another one is eggs 24 makes loads of mess but is easy to wash away.
or gellybaff. it comes pre coloured.mix 2 packets with a six inch full bath of water this makes it really thick and you can always add more water for thinner slime,or in bucket form one pack would make eight 5 litre buckets of thick slop or for thinner slime 14-16 its sold in toy shops or online
http://www.gellibaff.co.uk/
clean up easy again just disolve the slop with the powder included then wash it down the drain
hope this helps
bethany