Here are some tips.
Artist grade paints are more vibrant and pure because of the higher pigment content. Whereas student grade paints contain fillers like kaolin and have less pigment.
Try to use single pigmented colors to avoid mud.
One pigment = one color
Two pigments = two colors
When you mix two colors and both of them have 2 pigments, then you are actually mixing four colors, which may end up looking like mud.
Try to mix only 3 single pigmented colors. The more colors you mix, the muddier and duller the final color will become.
If possible try to use good quality paints that are either transparent and semi-transparent and try to avoid the opaque ones, which will also dull your paintings.
Clean your brush always in clear water whenever you change colors. Also keep two clear containers of water next to your painting. One container to rinse off your brushes, and the other container to apply clean water to your paint.