Mess is the cost of the activity, not a lapse.

Every rule you make to keep a session clean removes something the child came for.

Adults police mess for good reasons and then wonder why sessions fizzle. Do not put so much on the brush. Keep it on the paper. Do not mix those, you will ruin them. Wipe your hands. Each instruction is reasonable in isolation and together they describe an activity in which the child's real interests are all forbidden. Loading the brush heavily, going over the edge, mixing until it turns to mud and getting it on their hands are not lapses. They are the experiments. Prohibit them and you are left with a supervised colouring exercise that no child finds absorbing for long.

The workable answer is to move the problem rather than the child. Decide in advance where mess is acceptable and make that place genuinely tolerant: a floor covering that can be shaken outside, an old shirt kept for the purpose, a table that does not matter, a spot near a sink or a door that opens onto a yard. Then stop managing. The cost of the session is a wipe down afterwards, and that cost is fixed and known and can be budgeted for, which is much easier for everyone to absorb than a running negotiation conducted over the child's shoulder while they are trying to concentrate on something else entirely.

Bring the child into the clean up as an ordinary part of the routine rather than a punishment for enthusiasm. Brushes get washed, lids go back on, the floor gets wiped, and none of that requires a lecture about care. Children who pack up their own materials treat those materials better, and the ritual of finishing gives a session a shape. What you are teaching is that making things has an afterwards, which is true, and not that making things is a privilege that will be withdrawn if the table gets marked.