I just finished one with scrappy lights on the light side - the dark side was fruit and veggie fabric that was also scrappy. My grandson loved it!!

Thanks - now I have a plan in mind! Love the help and ideas.

You've seen a coupole here now that show a mixed light side of the block.
Here's yet another (shown on the board several times) that shows you can have every block different in darks and lights.

In fact, in this first case (with the pinks) there were some mediums tossed in on each side of the block, too, and it still works well.

The second picture shows a very controlled light/dark color placement.

Jan in VA

I, too, made an all blue log cabin
 

I, too, made an all blue log cabin

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I have also heard the red center represents the hearth of the home. Like Brenda Y said. Yellow can be used also.
I like the scrappy and floral log cabins. Play with design before you sew together, lots of possible arrangements.