For this sort of thing I have found chrome really useful , click mouse then view code, not at computer so cannot see exact wording, then change colors of things till you
Find the bit you want.  I changed that bar to a pattern to match my colour scheme

Sorry, I have the same problem reported by Redneck, i.e. I've placed it on the bottom of image pages but no similar images are shown (only the headline Similar Images). Could anyone help?

Thanss a lot :-)

I suspect you are using syxtra similar images rather than Leo's ? Plrang has instructions on his site for the syxtra one, help for Leo's is in this thread. 

Yesterday evening I have replaced Leo's Similar images with Syxtra Similar images.

But I want to solve the problem with Leo's one since I find them better in appearance. Still now if I insert Leo's one no similar images are shown, but only the headline "Similar Images".

Could anyone help? Thanks :-)

For Leo's plug in - have you activated the Related Posts Taxonomy plug-in ?

Now is working 
Thanks a lot for your help Christine!!!!

You are welcome Alessandro, pleased I could help.