Wow, am I having problems with my IMBC!  You all have such great insight, I am hoping someone can help me figure this out. 

When I make IMBC, it comes together nicely, tastes good, looks right.  All is well with the world.  I don't usually use my icing the same day I make it, so when I do go to use it, I let it come to room temp and whip it up a bit.  The icing breaks terribly at this point.  I understand it's likely to break, but this is so broken that it seeps and theres a good bit of liquid.  If I whip it long enough I can bring it back together, but it turns yellow.  I made it the other day and it looked fine, but when I went to scoop some into my KA bowl to whip up, it turned yellow just from coming into contact with the scoop. 

I have a friend who uses the same recipe, and I've worked with her icing and this doesn't happen.  Neither one of us can figure out what I'm doing wrong.

Now I'm turning it over to the experts....  Any help would greatly appreciated!!

 Buttercream is an emulsion (combining 2 liquids that don't normally mix) and like all emulsions extremes in temperature will cause it to break.   Fixing a broken emulsion is normally a matter of beating it very slowly for 10 min or so.  I use a paddle attachment for this.  I pretty much only use IMBC and never had a problem with it turning yellow except when I used a very yellow butter.

i found I get the best results with pure cane sugar and European style butters. I don't like the flavor and texture of the buttercream when made with American style butter.  I've found beet sugar to be unstable and inconsistent.  I once tried a cultured butter with 86% butterfat and thought it was greasy.

the ratios I found worked for me is 

1 part egg whites

2 parts sugar

3 parts butter

i base it on a 30g egg white

my standard batch is:

210g egg whites (beat with 1/2 tsp cream tartar)

410g pure cane sugar

630g European style butter

i heat my sugar to 240 degrees, and add the butter when the egg whites and sugar temperature drops to 85 degrees as the melting point of butter is 90 degrees.  I have a a large mailable ice pack that I wrap around the mixing bowl to help bring the temperature down. I beat on a very low setting  

while I always refrigerate my buttercream after I make it (I like it after it's been rewhipped) I know many pastry chefs who leave it on the counter as long as the room temperature is not above 75 degrees.  I've left it out on a few occasions and found it to be fine.

Well said! ^^ 

i, too, use imbc almost exclusively, and I just can't figure out the yellowing part.  I mean, it makes me want to ask if there was food coloring on your spatula when you rebeat it :) I can't think of another reason! 

if it breaks like that (looks like sugar syrup and clumps of butter - been there) it was too cold when you tried to rewhip it.  It must be fully soft and room temperature.  It can still be saved - while slowly mixing, warm the mixer bowl gently with a blow dryer until it starts to come together, then turn off blower, increase speed and whip.  Can use warming pads around the bowl, or very cautiously warm in microwave for a few seconds at a time as well.  I freeze smbc all the time, and let it come fully to room temperature several hours or overnight.  If I am in a rush, i cut it into  chunks and gently - a few seconds at a time on defrost setting warm it in my microwave, stirring and checking after each few seconds until it is fully soft then whip.  Good luck!

Thanks Norcalhiker!  Much of what we do is the same, but I didn't add cream of tartar I am not so patient when I rewhip.  I will try beating it slower for a longer time. 

Brookbakescake - Nope, no food coloring:)  Has happened before on cake when in contact with metal scraper and last time in bowl in contact with scoop.  After rewhip though, the whole bowl is yellow.  Not bright yellow.  It looks like butter.  Maybe it's a temp thing? 

Now that I think about it, seems like it must be in the making of it rather than the handling.  I handle my friend's icing the same way and it's worry-free.  Also, if it was just a temp thing, lots of people would have had this happen.  Will try a different butter.  Also, I take my syrup to 248, but not sure how reliable my thermometer is.  Will try a new one as well.

Yikes!  Unless yellow becomes a super-popular color for wedding cakes, I'd better figure this out fast!

 Would a tiny bit of violet food color take away the yellow?

That would just make it brown :(

i have made made it before with varying temps of the sugar syrup, doesn't make a huge difference.  If it breaks after thawing, just treat it patiently and it'll come back together.  It's an emulsion, like NorCal said, so just needs to be babied sometimes.  

Since the butter is what usually makes the most color difference in imbc (taking the meringue from bright white to pale yellow), maybe it is your butter that is reacting with something.

Also, what kind of flavoring are you using?

Now that I think about it, it is probably the breaking that makes it change color, and it's the drastic change in temp that makes it break.  Next time, let it come to room temp COMPLETELY before rewhipping.  

Let us know how it turns out!

Gosh, I hope you guys are right.  Thinking back, I don't think it was quite to room temp.  Yortma, I will follow your directions on rewhipping.  Can't wait to try again tomorrow!  It would be such a relief if it's just a temp thing:)  Will let you know what happens...  Thanks to all!!

I have had success dealing with refrigerated buttercream by warming about 1/4 to 1/3 of it in the microwave to a point where it has just melted (you have to keep a careful eye on it).  It should look like melted ice cream (it still is a bit fluffy but definitely pourable), and be a little warm to the touch but not hot. Then add the melted buttercream to the cold buttercream in your mixer. Use the paddle to re-beat it all and it fluffs up beautifully before long. Give it a try and let me know if it works for you. 

So happy to say you guys are absolutely right!   I let IMBC sit out overnight, and it handled perfectly.  Was from the same batch that broke and yellowed the last time.  Can't tell you how relieved I am that temp was the issue.  You've given some great advice on bringing to temp and rewhipping, and it will be such a huge help moving forward.

Thanks to all!!