@remus:
I am trying your advice but have some problems with it:
1.) I use the following procedure to project the image (from the sketchup docs):
A. Create a model, such as a cone, topography, or building front. This model will receive the projected image.
B. Insert an image using File > Import. The cursor changes to the Select Tool with the image.
C. Place the image in front of the model that will receive the projection.
D. Size the image so it is large enough to cover the entire model.
E. Context-click on the image and select Explode to turn the image into a projected texture.
When I do so, somehow the overlayed image gets semi-transparent. I can still see the original color of the object. When I delete it, then I get a semi-transparent object - the colored face seems to be behind the (flat 2D) object, no matter from which side I look at.
What am I doing wrong?

2.) The resolution of the exported image is quite low (I magnified the object as max as possible) so that I can clearly see the borders when overlaying. So, saving various images? Or are there other ways to get more resolution?

Thanks

Thank you Greyhead,
the K-tools helped. But I still have the problem, that the resulting object seems not to be joinable into an area.
What I want to do are scrolls (rolled rectangles). Any ideas?
Thank you for your time

Thank you TaffGoch,
unfortunately, this will work only for height differences. My model is much more simple, they are two 2D-coils with some straigt connections where I need the gradient.
I don't have Photoshop, so are there easy tricks (Ruby or directly in SU with the paint bucket tool) to succeed?
Thank you

I do not want a 3D helix, I want a simple 2D spiral (with the aim to push).
Are there any possibilities within SU? It should be much simplier to program than a helix!

No, not necessarily.
What I need is to define frequently spirals which I pretend to join by lines at the ends to get a helical area in 2D.

I tried to use a gradient texture. But I have curved areas and I only could find out how to stitch the texture in a specific direction (and not along the curves).
Any trick?

Hello,
I need to merge the color of two areas. I was thinking about using a color gradient. Is this possible are do there exist easier possibilities to change the color gradually?
I tried textures, but this did not work out (my final two areas are quite complex and I need the gradient starting from the end of one area into the other).
Thanks a lot.

Dears,
I am looking for a simple (Ruby?) routine to create a 2D spiral which is extendable. Something like define 2 circles, segments and number of rotations.
In the tutorials I found a lot about staircases and p.e. screw2.1 (which works nice, but is not extendable in 3D).
Any help?