Couple of reasons. It may look brown or black mixed and wet on the palette, but looks purple when dry. Also, the typical reds and blues are mostly transparent, try mixing in some white to make it more opaque and you will see the purple better. Also, more red than blue. Blue is a stronger pigment so it will dominate the mix. So... start with red. Mix in a little white. Add a tiny bit of blue.

It also depends on how light or dark your original red/blue pigments are and the type paint used.

Purple isn't made of just red and blue, there is more to it. Try using lighter reds and blues and add white so it can lighten it up. Most colors look darker or lighter when mixed and clumped together until they're spread out and dry on a canvas or what have you.

Hello It really has to do with WHAT KIND of red and what KIND of blue you use to mix the purple. It helps to understand some color theory, but let me try to simplify as best I can. all REDS are not the same... nor are blues... nor any color for that matter. Reds can be more ORANGY--leaning to the WARM side of the color wheel...cadmium reds for instance... and some reds are cooler...leaning more to a blue side...as in Alizarin Crimson, permanent Rose.... so the REDS that are WARMER..have more YELLOW tones in them, and the reds that are COOLER have more blue tones in them. Stay with me here... the same goes for the blues... some lean toward the warmer side...and start to get RED tones in them...as they get slightly more purple. some lean to the COOLER side..and have some green in them NOW...herein lies the problem of making brown. When you mix two complementary colors , it makes a sort of brown color. SO, red and green for instance...makes a rather dull brown color, and so does blue and orange...etc. If you pick a RED that has some ORANGE in it....and mix that with a BLUE that has some GREEN tones in it... you have just mixed the red and blue.... but you also have in the mix ORANGE AND BLUE and RED AND GREEN. VOILA....you have mud! Keep the blue on the RED SHADE...and the RED on the BLUE SHAde,,,and then they won't cancel one another out to make a grey/brown/black...and instead you will have a lovely truer purple. I hope that made some sense for you. Study a color wheel to find out more about the "cast" of colors and how they differ. All the best in your painting. Robin Miltner http://www.robinmiltner.com