With help of raandoom, I created a script what can easily change boot logo and background color directly from N900.
Usage:
logo.sh [ -h ] [ -l logo ] [ -c color ]
-h: shows this help.
-l: flashes an boot logo. File must be 16bit BMP. (Example of saving in GIMP)
-c: set backgroud boot color.
Color must be in framebuffer 16bit format.
It consists of two bytes: RRRR RGGG GGGB BBBB, where RRRRR, GGGGGG, and BBBBB — values of red, green and blue colors.
And these bytes must be swapped because of little-endianness.
So, for example, if you want to code red color, it will be 1111100000000000 = 00F8, blue will be 1F00 and green will be E007.
Result:

Here the link: https://hxka.keybase.pub/maemostuff/logo.run
It should support all of RX-51 revisions, but if something would go wrong, you can always use Cold Flashing.
Thanks raandoom, Xlab and tz1.