Hey, when you have questions about NBitcoin, I think posting on github is better, because I will see it more easily.
You can reach me by mail as you finally did, however the info I gave you won't be indexed by search engines. :p
Responses for other people that might find this question :
* Open Asset is not SPV compatible, which mean you need either need to trust third party services or having a full node and scanning the blockchain yourself
* You can ask a proof to third party services for colored coins, the problem is that the proof can be arbitrarily big, especially with OA.
* QBitNinja is a wallet service which support OA, and open source I created.
* The key management and signing must be done by the client with the libraries you mentioned. (QBitNinja can only generate new addresses, but you can do it yourself client side and add to your server wallet)
The public server of qbitnina (
http://api.qbit.ninja/) is not meant for production so you would need to host it yourself.
If you don't want to bother, and don't need the wallet feature of qbit ninja, then Coinprism is a better bet.
Since the client can't ask proof to third party server about colored coins, you should cross check the coins you are spending in every new transaction are right with several explorers. (coinprism + qbitninja)
I think chain.com is working on OA also, I have seen some commits on their repo... not sure about it though.