I love this way of  mark making!!!
I try to do this with Procreate and made a marker brush.  I have two problems. When I I increase the opacity of the bru sh, the colors are more vibrant, but there are no secondary colors where two colors met. With less opaque brushes there are secondary colors, but all colors are dull.

I had to find a solution and will get real life brushes to do it on paper! So I am not finished yet!.

For procreate I finally used for every color a layer and  adjusted the opacity off the layers with the yellow layer on top and red between yellow and blue. I duplicated the lager and snapped them together. Then I duplicated the layer for intenser colors. Blend modes gave fun effects too! Some playing with hue saturation and brightness give the final more colorful results!