You can paint hardboard yourself after laser cutting. Its surface and dark laser-cut edge can absorb paint differently, so one coat may not produce the same result everywhere. A small test piece shows what preparation your finish needs.
Start with a clean surface
Remove dust and loose residue without soaking the sheet. Hardboard is a fibreboard, and excess moisture can affect its surface or shape. Sand only where needed and remove the dust afterwards.
Treat the edge as a separate surface
The cut edge is darker than the face. Decide whether it should remain visible or match the paint colour. An opaque finish may need a different preparation or another coat along the edge.
Painting internal edges takes more work on an open pattern. Consider that when drawing many small cut-outs.
Test primer and paint together
Follow the paint manufacturer's instructions for fibreboard and try the complete combination on an offcut. Assess coverage, adhesion and warping once the layers are dry. The appearance immediately after painting is not the final dry result.
Paint before assembly where practical
Loose parts are easier to reach from all sides. Keep glue surfaces and precise joints clear where a coating would affect adhesion or fit. Assemble a painted test part before finishing a complete run.
See hardboard and available thicknesses. If you are choosing a sheet for a painted part, compare hardboard and MDF.