I've been making art for a while and I keep running into the same issue: we don't have brown, grey or beige. Here's an example:
As you can see, this burger has lots of brown and beige. To convert it to Haven colors without dithering, the brown must be replaced with orange and the beige must be a mix of red and white, orange and white or fuschia and white. Maybe I could make it look better with more editing but it will always look slightly off because we don't have the right colors.
Another example:
There's no way to shade the black half of Monokuma because we don't have grey. Everything on that side would be black and it would ruin the muscular effect.
So here's my suggestion:
Pink/beige pigment could be crafted with strawberries and bone glue.
Grey pigment could be crafted with ashes and milk.
Brown pigment could be crafted with mulch and bone glue.
Other pigments would be nice but just getting these three would be a good start.