It's really not all that hard. If the rules to the lineart say that you can use it to make free adoptables, then you can color it in and adopt out the characters/designs you create. I know that this lineart can be used for this purpose because I've used it myself, see:

Here are a copy of the rules for the lineart:
Do NOT remove my name.
Do NOT sell for profit.
Do not collect $200.. I mean..
Do NOT ask me to Draw/Color/etc for you because I really don't have the time.
(I barely have time to draw for myself.. so I really don't want to do requests/commissions)
You may adopt out the designs, but it must be for free.
So you can adopt out the designs. :3