Prepare paper that tolerates water
Use watercolour paper and test a scrap before folding a complete card. Office paper can buckle badly or tear under repeated brushwork. Prepare two related colours, a reasonably broad brush, clean water and a table covering. Keep a dry space for the eventual message.
If taping the paper down, choose a suitable tape and test removal on a scrap. Some surfaces tear when tape lifts. Alternatively work on a separate sheet that will later be mounted on the card. This avoids wetting the fold directly and lets you select only the portion you like. Keep the mounting card out of the wet working area.
Make a pass instead of constantly correcting
Prepare a small pool of diluted paint in the palette. Work on dry paper for a defined edge. For a diffuse effect, lightly dampen the area before adding colour without leaving an uncontrolled puddle. Set down the brush and move steadily. Water quantity matters as much as pigment.
Add the second colour while the first area is still damp if you want them to meet. Then let them move. Repeated brushing can mix away the differences and wear the paper surface. If a bloom develops, observe it before trying to remove it: it may become part of the design rather than a defect.
Wait until everything is dry before judging the colour and deciding what comes next. Wet paint often looks different. You might keep a large white area, add a small drawn line or write one word. Choose a compatible pen and test it on a scrap painted in the same way.
If the sheet has slightly buckled, use a flattening method appropriate to the paper once it is dry, without sticking the painted face to another surface. For a mounted version, use compatible adhesive and let the card dry flat. The result need not be symmetrical: it depends on a readable contrast between wash, white space and message, not on removing every trace left by water.
