Squirt some red acrylic paint onto a scrap piece of cardboard or use a lid from an old food container.
Squirt an equal amount of blue acrylic paint next to the red. Leave some room between the two colors for blending.
Dab your brush into the edge of the red paint, pulling the red toward the blue. Dip the brush into the edge of the blue paint and drag some of the blue toward the red.
Mix together the red and blue where the paints meet, using your brush to pull in a little more of each color as you go. If you still notice pure red or blue on your paintbrush, wipe it onto the cardboard and mix the colors some more. Keep mixing the two colors until you have a workable amount of violet or purple.
Paint a bit of the violet onto a scrap piece of cardboard or a clean section of the old plastic container lid to test the color. Add a little more red if you'd like it to be more of a red violet. Add a little more blue to make it more of a blue violet or indigo.
Add a very small amount of white or black if you need to lighten or darken the violet; the smaller the amount, the better, for starters. Do this only if you feel it's necessary, as it will permanently change the color.
Paint something with your new violet acrylic paint.