First, you need to have lots of patience and time.
Then, try to get 2 boxes of 25/25/25 black or 230/230/230 white.
Now, get colors (colorful ones) in the rest of 4 boxes, you can go for 3 if you don't want to do this anymore. But don't left 2 boxes for brown/greys, that's 1/3 chance of getting those shitty color....
Here's an example:
The lowest one can probably sell for 3m, that's 3m guaranteed, while the highest one go for 12m.
And don't sell those dyes now unless you're really in need of money, the sale made many people stocked up and the priced is lower than usual.
Enjoy your money making!!