but I'm still wondering what's the issue with the colour...

There's really no issue:
The reflected colours change as the light changes – just as in real life.



To best manage colours, the first thing you should do is to calibrate the brightness of your 3d view,
if you haven't done that already:
Apply a completely white texture to the sky.
Render different coloured objects with [font=courier new]enhance external brightness[/font] checked, and no sunlight.



Adjust the brightness of the 3D view until the brightness is as close to the rendered image as possible.

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I find that applying a white top to a partly clouded sky image, is a good way to get nice daylight colours when using the enhanced brightness option.



cec