my poor grandmama :(

My keepall 55 didn't come with one either, oh well!!! I have read the best thing to store them is wrap the keepall in a cotton sheet - so it can BREATHE!!! That's what i plan on doing, although right now my house is for sale and I don't want people stealing it - so I take it to work with me everyday and it sits covered up in my car - kind of funny, people keep seeing me come in and out of my house everyday with my keepall - one can only imagine what is going through their heads! HA! HA!
 
I would be afraid the color would transfer too. If it were me, though, I would profusely thank her for her time and trouble and then just let it go at that. I wouldn't let on that I couldn't use the dust cover.

Maybe you could use a kind size pillow case (in white, of course), as a dust cover?
 
I would not use it!! Even if you wash it and it doesn't bleed, I still wouldn't use it. Just tell Grandma that it's the perfect size and you LOVE IT but the color is too dark. I believe in being honest. I'm sure she will understand if you explain to her that it's just due to the fact of the color she used. I also agree with your mom that you should send her the fabric you want used and ask grandma if she wouldn't mind making you another one.
 
Can you send her some white flanel fabric, they type you make baby blankets with? You can get it at any fabric store. I would call her and explain to her how you need it in white and I would send her a little gift for her being so nice to you. I am sure she will understand and I doubt she will be upset about it. To me it is better to be safe than sorry.
 
I highly doubt it will transfer - it's not like it's going to be rubbing against the bag. But then again I don't know. Anyway, you have a sweetie of a grandma.
 
Send your grandma lots of love and thanks. Wash it a few times and it should be fine. Or take it to a local tailor and have them put a liner in it. That way you will always think of Grandma when you see it but not worry about your bag.