Just posting some information -
Interesting observation, lomateron. It apparently has nothing to do with shaders as I can get the same type of change rendering just the backbuffer cleared to the same color for both R16xx && R8xx as you have observed.
Note that the R8xxUNORM is a bit pattern format, whereas the R16xxFLOAT is, well, floating point.
I'm not claiming to understand why it occurs - just noting that the docs for ClearRenderTargetView (which takes a 4 float array for the color) states:
Applications that wish to clear a render target to a specific integer value bit pattern should render a screen-aligned quad instead of using this method. The reason for this is because this method accepts as input a floating point value, which may not have the same bit pattern as the original integer.
That implies that the bit pattern for integer formats created from the floating clear color may not produce the same color.
Just FYI: clearing the backbuffer with clear color = (0.0f, 0.125f, 0.3f, 1.0f), doing a screen print, opening Paint, ctrl-V and sampling the color yields:
R16G16B16A16_FLOAT
with clear color 0.0f, 0.125f, 0.3f, 1.0f
Hue 133
Sat 240
Lum 70
Red 0
Green 99
Blue 149
DXGI_FORMAT_B8G8R8A8_UNORM and DXGI_FORMAT_R8G8B8A8_UNORM
same clear color
Hue 143
Sat 240
Lum 36
Red 0
Green 32
Blue 76 (blue 77 with DXGI_FORMAT_R8G8B8A8_UNORM)