This is actually a multifaceted question as it really depends on how many MIMP you have, whether they are painted or not, and what temperature you intend to store them at. I'll start with quantity:
Red Faded NY Hydra (Check the toes)
If you store them, as I used to, in a large chest, make sure to separate the colors in some way. Ziplocs or grocery bags work fine (subject to one limitation below), or you could use dividers of some sort.
Now for the next consideration: the heat.
I have always thought that heat can cause color bleeding. However my recent experiments have not shown fading among single monsters touching in 90 degree (32 celsius) has not caused any fade over the month they have been touching.
The only thing that I have noticed is that ziploc bags tend to make the figures sticky if the temperature gets too hot. So if you intend to store them in heat, I recommend that you use something other than a ziploc. This especially seems true when the figures are painted. I find that mild use of alcohol or dish soap removes the stickiness (be careful not to rub too hard or you will damage the paint).
Painted figures have multiple colors and it has not been proven that they bleed like the other figures. I intend to test this eventually, but I have not done so yet.
I hope this helps some collectors out there with storage, and please leave comments or questions below. If you pick out any spelling or grammar errors, feel free to post those too and I will edit the post to fix them.