Hi Wildflower! I don't wipe my bowls/utensils with vinegar. It didn't look like Haniela (in her vids when she mentions vinegar) washed/rinsed after wiping with vinegar. She only wiped the mixer bowl with vinegar.
Yes, plastic is porous but I have not incurred my icing cracking... (I get craters now and then - lol.) I use Wilton.
Maybe the RI ratios are off and that caused cracking. As for coloring - it will darken as it sits/dries and can continue to darken. All products don't contain the same ingredients. (Hope I said that right.  And some colors, to me, seem to make the icing thicken. I seem to have that problem with Duff's. Maybe I'm wrong... But, the combination of colored products (gel/liquid/concentrated (Wilton garden set) in an unbalanced icing ratio may factor into cracking.