In general, inpainting is not as central to the conservation treatment for works of art on paper as it is for easel paintings.
Watercolor is often the medium of choice for paper conservators during inpainting, but it is not reversible without a barrier layer. During a WUDPAC seminar we tested a variety of inpainting materials including dry pigments applied with a stub, dry pigments in aquazol, colored pencil, and graphite.
The verso shows my notes of what i used in each corresponding area of the recto.