It's been driving me mad but i remember where i've seen that repulsion and propulsion paint idea before, from the folks at
Digipen! Tag: The Power of Paint was a Digipen project released for free a few years ago and worth a look not much of a game but still surprisingly fun for a student project
No surprise really as the original idea for Portal came from students at Digipen

(then called Narbacular Drop) Good to see Valve giving more of these guys a chance of put their cool ideas out to a bigger audience.
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-You know what they say about love and war.
-Yeah, one of them involves a large amount of physical and psychological pain and the other's war.