technique please. i am not very good
Okay let's use your RWB render as an example.
If you're using crayons, I suggest a light cross hatch. Avoid using circles, and avoid applying a lot of pressure. Have patience and keep going until you reach your desired depth.
You want to visualize where your light source is coming from. Once you've set a light source you can then visualize your shadows.
Colouring a car comes in various gradients. A car is not just one solid colour. So try to get as many gradients of a certain colour.
Back to the light source. On your RUF, based on your shading, it looks like the light is mainly above the centre of the car, offset a few degrees behind the driver's side fender.
If you're using colouring markers, use straight and curved lines to define the car's body lines. Go over the lines a few times until you reach ypur desired depth. Think of how light cascades.
Use your deep colours to define edges, body lines, and shadows. Use your lighter shades to blend between gradients.
In regards to blending, if you're colouring your passenger side fender, take your lighter color over your darker colour and fill it in upwards toward your hood.
You've got the basic idea of shading so just replace the graphite with colour.
All in all you just need practice.
With oily hardened materials, I prefer a cross hatch. I avoid the use of circles for major body panels or on cars that are mainly straight. I use circles for shading head light housings or any circular part on a car.
I'm sure there are more experiencedand capable artists in thisthread that can give more in depth details about this. Especially when it comes to using markers and acrylics.