PAMMU
Adobe photoshop where photographs can be brightened or vice versa and altered.

Kobra
the paintings by Osnat.

Puppy Zwolle
Better pigments means better colors. Also varnishing brings out brighter colors.

Nima
You can use an acrylic medium alongside water. see here: and by "medium" I don't mean the material which we use to paint; it's something which you can use to alter or enhance the characteristics of your color. also, you can varnish your painting when it's finished and completely dried, that would help a little to make it brighter. but, my recommendation would be working with oil colors, using refined linseed oil as your solvent and varnishing it when it's finished, it'd be damn bright!

Sport
You need to throw in some darks for contrast. Take another look at the Osnat paintings. Colors in Osnat's paintings look so much brighter because Osnat uses a great deal of dark colors to bring out the contrast. My painting teacher used to say, " There is no loud without soft and no bright without dull". Put some darks into your paintings and your bright colors with pop with vibrancy. (It is called the Haptic Illusion) You could even use ultra bright day glow paints and if you didn't throw in any dark or dull colors for contrast you would see that the bright intensity for all the colors would just be canceled by the other equally bright colors around it. You just need to throw in some darks for contrast.

redladyart
Acrylic gel medium will brighten your acrylic paints. Simply brush a layer of it over your dry painting. You can purchase it here: Or use varnish that is compatible with acrylics.