That's the nature of transparent colors - their color varies based on thickness (brush strokes tend to leave ridges of various thickness, and thus darkness). You can try doing whatever you can to make the application as even/smooth as possible. Try a wide, soft taklon brush with a light touch and a single stroke (going over and over might make it more streaky). I would expect some acrylic mediums would/could allow smooth marks...especially a flowing/leveling medium. If you want to avoid dealing with streakyness altogether, you can add white to opacify or just use more opaque colors.
I've learned to appreciate that streakyness...it's the opposite of 'flat-looking' and it can be very beneficial and aesthetically pleasing for underpainting by allowing edges to intermingle, avoiding hard flat blocks of color.
Color is the most important element in painting - except for everything else
Last edited by Patrick1 : 02-08-2018 at 04:29 AM.