When my daughter got married, I volunteered to paint pebbles as place markers for the tables. We got smooth pebbles from the beach, washed and dried them, and then I painted 80 (!) pebbles. I used acrylic paints, which are water soluble and can be painted over if you want to change a bit. I was advised that varnish wouldn't actually enhance the appearance so I just left the paint as it was. You can get gold and silver metallic paints to give a bit of a sheen if you want. I generally tried out the design on paper first; then I would just brush the outline onto the stone with water, to see how it would all fit on - the marks lasted just long enough for me to block out the design with paint.
I painted each person's name, and then tried to personalise the pebble with something relevant to them: a musical instrument, an animal, a flag, a national emblem (Welsh dragon, thistle), a hobby (football, Minecraft) etc. If I was stuck I just painted something pretty like a rainbow. People loved them and photographed them and kept them as souvenirs. I really enjoyed doing them and it cost next to nothing.
My grandkids wanted to try it afterwards so I let them use my paints on some spare pebbles. Grandson (age 7) painted the whole stone black and said it was "a bomb". He gave it as a present to his best friend!