In a vacuum the coloured particles would move very quickly... why did they move so much more slow in your solutiin.

Answer 1

not sure the background, but . . .

in a vacuum there is no pressure (theoretically)

the particles would move more slowly when there is pressure, i.e., when there are other particles packed around the coloured particles inhibiting their movement

Answer 2

There is no pressure in the vacuum to inhibit movement so the coloured particles move very quickly