This orange, later red color is caused by some polymer which forms when the reaction runs too long! So better next time make a shorter reaction, it suffices with a lot less time needed.

also baddawg mentioned this polymer forms after filtering in his mother liquor, this is because there is still active catalyst in it. you can use less catalyst too.

butylamine is one of the strongest and fastest catalyst for nitropropene preparation also.

Sorry to bring this back from the dead, but I noticed something was never mentioned here which I think is important if one wants to obtain good yields in this reaction.

The precipitation of product is driving the equilibrium forward by removing nitroalkene from the reaction mixture. Even if the entire reaction is run at room temperature for weeks it will never go to completion without intermediate crystallization(s), and the orange polymeric byproduct will inevitably form.

That being said, it will be very hard to get the nitroalkene (I'm writing primarily with regards to phenyl-2-nitropropene here) to crystallize out of an incomplete reaction without first having some seed crystals. If no seed crystals are available it's probably necessary to push a very small scale condensation to the orange/red stage just so you can precipitate some crude crystals to seed future reactions. Once you have some seeds on hand there's not really any one set way of going about it- just add a bit of heat to get some product forming, seed it out perhaps over multiple crystallizations, then apply a little more heat if the product stops coming out and repeat. This has been the highest yielding / cleanest approach in my opinion.

If you don't have seed crystals you'd probably be better of scraping the vial

Just dont clean the glassware it is supposed to crystallize in like you were your mother, some waterstains are enough to get it going 

Experiment showed that it is not about the absolut maximum temperature which should not be exceeded but the time. Quick heating to 80-90°C in the microwave does not induce orange color but if it is kept very short, just heating up and done it stays greenish yellow as it should and....

Yes and. It was discovered that when doing it in the microwave it is frigging easy to determine the right point to stop because the greenish-yellow clear solution turns turbid, the greenish has vanished and its yellow alone. Thats the moment to slowly put it in some icecold waterbath and crystals crash out instantly.

Thats what has happened by now and we will see if the filtrate later will yield further crops. Now in the fridge to get all first crop crystals to form, then filtration and on for more. Will there be more? Thats the question which will decide upon the fate of this way.

If Elektro`S and Gusie are right and this step by step harvesting works, it will be the most simple reaction ever. The actual reaction time in the Micro was less then 2 minutes. There are good chances IMHO the speed of the reaction prevented any catalyst degradation.

Benzaldehyde and Nitro were fresh distilled. This seems a must for a clean reaction. Without there were low yields and plenty of problems.

/ORG

Much more interesting IMHO would be the effect of different bases on the stereoisomeres, metal hydroxides give IIRC a not so favorable distribution compared to triethylamine. But how do primary amines fare? Somebody knows?

A late reply but...
http://www.scielo.cl/scielo.php?pid=s0717-97072004000300013&script=sci_arttext