Vellum because it is more like painting than working with the markers on regular paper. I'll step you through my entire process so you can get a feel for how I illustrate.
I began with a pencil sketch of some peaches based on a few photos I looked at on the internet. I decided to go with clusters of peaches still on the branch, rather than peaches on a flat surface. Then I quickly outlined the main lines with a multiliner and erased my pencil.
You'll note that the image is reversed from the finished work. This is because I am coloring the back-side of the vellum from what I want the final to be. My colors look more intense on the reverse and more streaky. On my final work the colors look more washed together because I scanned the front of the vellum. (If you are doing this technique you don't have to do it reversed).
Next, I taped my Vellum to the background paper so the image wouldn't shift.
When working on vellum I like to do the largest areas first, in this case the main background color-wash. I added a few streaks of W3 and W1 in the top of the image and I added B97 and W1 to the bottom for my background colors. I then took my sketch colorless blender and washed the colors back and forth in long, even strokes. I was OK with this being streaky since I wanted my final to look more paint-like than usual.
Notice how I am not worrying about going over the lines into my leaves or peaches. I clean this up next. I grabbed my juiciest colorless blender and erased all the areas I didn't want colored yet.
On the larger areas I used the chisel nib to erase. I start in the middle of an area I want clean and I swipe up any marker on the vellum. It gets picked up onto the tip of the colorless blender, so I have to scribble it off onto scratch paper or a paper towel. Otherwise I would just be smearing the colors around more.
Slowly but surely I erase any areas that I need clean. I use the fine point on my smallest detail areas last. Now I am ready to add color.
I want my leaves to be fairly bright green, so my base green is G14. The natural shadow for G14 is G16 and then I need a really deep, dark shadow for the most contrast so for the darkest areas I use G99. Remember, working on vellum is more like painting, so I know that although my first strokes of G99 look all wrong next to the brighter G16 once I go back over them with the G14 then it all looks good together.
I also tend to work dark to light when blending on vellum. I add my deep G99 shadows, streak in some G16, then go back with my G14 and in smooth strokes I blend all 3 together. I work in the direction of each leaf along it's spine so that any streaks I may have will flow naturally together.
Now my colors seem to have flattened out and smoothed together. If you lose too much contrast just add more dark back in. The beauty of Vellum is you can erase completely, so a mistake is never permanent. I also will be deepening my shadows later, so I'm not worried that I lost my darkest G99.
Note how I am making sure that leaves far away are darker than the leaves in front. Always be aware of your shadows so you don't have a big shadow problem when you are done. On a larger illustration like this I strongly suggest doing a shadow study first.
On the bright green leaves in the foreground I got the shadow simply by layering the G14 twice. You get much more color range from a single marker on vellum than you do on regular paper. You'll also see some slight veins in the leaves. After I colored the smooth main color on each leaf I went back in with my middle green, or G16, and lightly drew in the veins- not too much, it more of the hint of veins.
Last I add E55, E59, and E49 for the branch. Again, streak in the direction that the wood grows for it to look most natural.
If you look at this comparison you will see how the side I am coloring on looks richer and darker, but it also looks too crisp for the effect I want. By flipping it over you get just a bit more softness and the colors look smoother (also, the camera picks up the gloss on the colored side more than the matte finish on the reverse).
It is all a matter of personal taste, so don't think you HAVE to do it the way I do.
Now our picture is really starting to take shape. Looking back now it might have been easier to do my peaches first and erased the leaves back out. Oh well, maybe next time.
Just like on the leaves, I added my deepest shadows first with the YR07 and E99. I blended these together in curved strokes (to mimic the curved peach surface) with a little E97 and mostly E95. On paper these colors would be way too dark, but on vellum you get more subtleties out of your middle/dark colors.
On the side of the peach closest to the light I removed extra color with the colorless blender. Sometimes when you layer too many colors on vellum you get a bad build-up of excess color. It's a good idea to clean it up as you work so you aren't muddying your blends.
Keep your strokes as minimal as possible if you want to avoid streaks. Again, I wanted a more paint-like look so I don't mind the streaks like I usually would.
After I colored and blended each peach I added my final layer of deepest shadow with a E19.
You might be confused why I'm mixing E's and YR's on my peaches. On vellum, most of my YR markers were too light, so I had to go darker than usual. If you darken a Y or a YR too much then you end up getting browns, or E's. The color families in the E's that I am working with are very reddish- The E90's are all orange browns and the E10's are reddish.
Last, I color the markers with T5 and T6 washed together with the blender. The only other color I used is the R59 for the tip of the red marker.
Adding darker shadows
When I finished working the back side I flipped over my picture. I want to deepen some shadows that were lost, but I also want to keep my lighting and shadows consistent. So from the front side, or the un-colored side of the vellum, I added shadows under the markers and on the leaves with B97, the same blue that I used on my background. This pulls all my colors together and it cools down my shadows.
Here is my final artwork again, scanned and color-corrected (much better than the in-progress photos). All the little subtleties came together and make a good whole. I see a whole bunch of problems with this art, but I know that I will always be my own hardest judge. I hope this helps you get a good idea of how you can also get a "painted" look with your markers on vellum.