I don't want to buy colored fondant, I'd rather color it myself. But the one time I've colored a tiny handful of fondant, it took forever!

Any tips on doing a large amt? Enough to cover a 3 tier cake? Without all being slightly different shades?

add color when first melting the marshmallows.

I think it also depends on the coloring you use. Something that you can actually count the drops. I use a brand called spectrum. And have no problem making large amounts of one color and it being consistent. I've been able to make some deep reds with no problems.

If I'm colouring a large amount for a tiered cake, I divide the fondant into 3 or 4 equalish blobs and colour each blob separately and then knead the whole lot together at the end to make sure it's even. HTH!

I divide the fondant into two piles. Color each one and then divide each one in half and kneed the two opposite piles together. Keep dividing and kneeding opposite piles until well mixed.

If you make MMF, I know you can add it to the liquid part (before adding the powdered sugar). I believe other homemade fondants work the same way.