Now that you've got a growing collection of full-size helmets, you need to start protecting them from dust and curious fingers.  Whether you buy an off-the-shelf case or have one custom-made, you can still customize and personalize it.  I prefer cases with wood bases that have an acrylic cover slip over the top.  I like to paint and detail these wood bases for each helmet, and it's amazingly easy to do.

Sand the wood base down with some 150-grit sandpaper to give a nice, rough surface for the primer to adhere to.  Clean the base off and then spray 2 or 3 coats of primer on the base.  I prefer to use white primer, as it tends to not interfere or bleed through the paint colors going on later.  Give the primer a week to fully cure, then you can apply your base color coat (start with the lightest color, and apply darker colors over the lighter ones to prevent bleed-through and cross-contamination).  Give the color coat at least a week to cure, and if you want to apply subsequent colors, use masking tape to mask off your design.  Spray the subsequent darker colors and allow to cure for at least one week.  After that, I use paint pens to accent the colors and separate the sprayed colors (above, I used white to separate the blue and yellow; to the left, I used black to separate the red and gold).  Afterwards, I seal the colors with either a clear gloss enamel, or glossy polyurethane.

I use just basic enamel primer and paints, but that doesn't mean that you can't use anything exotic.  Don't be afraid to think outside the box and experiment.  You could even color-coordinate your helmet base/case to match your home or furniture colors, or whatever.  I've had great results from metallics and pearls, especially when sprayed over silver or gold.

For paint pens, I use whatever is readily available.  These are available at many craft and hobby stores, and are not very expensive.   Make sure you clean the tip with a paper towel after use, and store right side up.  Keep a piece of cardboard handy when using paint pens, as you may need to occasionally clean the tips or check for paint flow.  I typically use a steel ruler to align the paint pen when using them, and apply multiple coats (especially when using the lighter colors over darker base colors).