Noticed some oddities while trying to do testing for a different post. Reduced it down to easy to reproduce steps. What the heck is going on?
Using Photoshop CS6 v13.0 x64, on Windows 7 Ultimate.
1. Create new document - could (probably) be any size, but I chose 1000x1000px, 720dpi, rgb/8, ProPhoto RGB.
2. Open color picker, choose LAB 60/-60/-60. Should also show RGB 67/144/237.
3. Fill the entire document with this selected color.
4. Make sure proof setup is custom, set to SRGB IEC61966-2.1, Preserve RGB Numbers *un*checked, Relative Colorimetric, Black Point Compensation checked.
5. Turn on gamut warning. As expected, entire document shows out of gamut.
6. Convert to profile sRGB IEC61966-2.1, Adobe (ACE) Engine, Relative Colorimetric, Black Point Compensation checked, Use Dither *un*checked.
7. Since we converted profiles, everything should now be within sRGB color gamut, right?...
8. Turn on gamut warning. As expected, none of the document shows out of gamut.
9. Eyedropper the document's new color, open color picker, and it's at LAB 66/-17/-50, RGB 0/173/251.
10. BUT WAIT! There's an exclamation point in a triangle indicating the color is out of our proof gamut.
If we click the triangle to select an in gamut color, it selects LAB 64/-17/-37, RGB 64/167/222, which is significantly different, and now color picker is happy.
Why in step #10 is color picker indicating the color is out of sRGB gamut, when step #6 should have ensured it was within sRGB gamut?
