The reason the appearance of the whole spread changes when you add a transparent object is because to handle the transparency correctly, InDesign has to view all the objects using the same color space. InDesign is a bit unusual because it can include both RGB and CMYK objects in the same spread. But when you add transparency, it looks to the transparency blending space for the document (found in Edit > Transparency Blending Space). For a print document, that would normally be Document CMYK. For a Web or Digital Publishing document, that would be Document RGB.
thanks Steve for your reply. but i don't think i understood how the color profile contributes in changing the image.
im sending attached two print screens in case its easier to understand the issue.
in the first image you'll notice that there's a vertical black transparent object on the right side. the image is also lighter [which happens when i place a transparent object in the layout].
in the second image you'll notice that the image is darker, as the original jpg file, because i took out the transparent object on the right.
I'll let our color management expert Rob Day provide more information. He'll probably stop in here tomorrow.
I do know that InDesign doesn't have a real grayscale color space, unlike Photoshop. David Blatner, a real InDesign expert, created a good series of videos on color management in InDesign on Lynda.com. He says, "To InDesign, a grayscale image is either defined as CMYK or RGB. If the Blend Mode is set to CMYK, then InDesign thinks of this as a CMYK image with data only on the black plate. If the Transparency Blend Space is set to RGB and there's transparency on the page, then internally, InDesign displays it as though you converted it to RGB in Photoshop."
But I'm not enough of a color management expert to explain it beyond that.
thanks Steve, that really made it clearer to me. so i just tried placing the bw image in the transparent blending space as rgb, in step of cmyk by default, and it automatically assumed its original lighting/contrast. so problem solved.
i will however see if David Blater or any other expert in color management give any reply to better understand how this happens. but at least i now know the solution. so thanks again!! =)