Orcs are usually loyal to the Empire. In fact i recall when i was playing my Orc, a group of Leigonaries with a prisoner went through a Orc Stronghold (the one near Markarth). They opend hte gates for them, and even greeted them in the traditional greeting method. Even though one of the Orcs was wearing Stormcloak officer gloves.

lets look at who the Nords are "racist" towards

The Khajit caravans.
To be fair they are drug dealers(They are all holding as it were) and more importantly the Khajit are part of the Aldmeri Dominion.Ask yourself how a Breton caravan would fair in Summerset right now.

The Argonians
The ones in Windhelm in particular
They are not allowed to live in the city because there is a large population of Dumner already there.These two groups are having issues ATM and Ulfric does not have the manpower to maintain peace between them.I am not saying the situation would blow up but the possibility is there.

the dumner
or "The house guest who would not leave" it has been around 200 yrs since the destruction of Morrowind and the subsequent curb stomping by the Argonians.The dumner have to make a decision.They claim this isn't thier fight because Skyrim isn't there home well then take back a town something anything.Go to the border step over and proclaim it Dumner territory again,if no Argonians show up take another wash,rinse repeat.If they are not willing to do so the Nords have every right to expect some support from the Dumner.They dont have to join the front lines but something anything as a show of good faith.

The Empire
All the empire has taken away from the Nords is freedom of religion,the right of self rule and the legal system not ideal (really death sentences for border crossing and gate opening)
The empire is willing to use the awesome power of the empire to defend the provinces as long as it is named Cyrodil(Ask the dumner ,orcs ,and Redguard about Imperial support when they needed it)
At least the Empire does not allow a hostile foreign power round up citizens in Imperial territories for questioning and execution.Oh wait they cant even do that

The Thalmor
No explanation neccesary

If the Thalmor consider you undesirable death squads are sent to find you where ever you hide
If the Empire considers you undesirable they take a little off the top
If the Stormcloaks find you undesirable they send two drunks to yell at you late at night

so yeah Stormcloak and Nord racism is fairly overstated but simple ideas for simple minds

I believe, with some imagination, you can come up with a good plausible RP reason for ANY decision you make game-wise. An Orc could join the Imperials because he knows that the second the war is over and he is of no use to them anymore, they will turn on him because he's not a Nord. That same Orc could join the Stormcloaks purely out of anger that the Imperials dared try to execute him for no good reason. And you could probably come up with many more reasons for your decision either way.

this