Thank you guys.
I have been playing with stain and dyes for a couple of months and I am still learning.
For the gradient, after properly sanding, I just started with a black stain in the airbrush. I brought it down using some 0000 steel wool.
Did the same with the red stain that, having a white background, went towards pink.
For the cases I use maple branches with a diameter larger than 1 inch; sometimes oak.
Usually I roughly turn one part of the case, bore it with a forstner bit (I think the 3/4 of inch diameter) around 7/8 cm deep (depends on the pen style). Mount the other part on the lathe, do the same (using a smaller forstner bit, I think the half inch) and make half inch tenon to have a tight fit so I can turn the case.
I finish them separately just using scrap wood pieces mounted on the scroll chuck with 2 inches tenons.
I sand the inside with a little drum mounted on the drill press, then apply some lacquer to avoid any dust to stick on the pen. After the lacquer is cured, I glue some foam inside at the bottom of the two ends of the pen case. This will avoid the pen to travel around the case.
Please excuse my English.