The out of gamut overlay is a legacy function before Photoshop could actually show you a soft proof and thus, isn't all that useful. Its supposed to be showing you all out of gamut colors based on the current setup for profile and rendering intent but it treats everything equally (just a little out, or a lot out of gamut).
The overall color should not change unless you're also turning on and off the soft proof.
Soft proofing is a great leap forward over the out of gamut display, but soft proofing is still limited by the gamut of one's monitor. Many monitors can not display much beyond the gamut of sRGB and this obviously limits softproofing beyond the sRGB gamut . For example, here is a screen capture of an image in ProPhotoRGB containing saturated reds.
With soft proofing set up to simulate sRGB output, the out of gamut display in Photoshop shows that nearly the entire image is outside of the sRGB gamut. My monitor is not wide gamut, so there is little difference between the displayed and soft proofed image. The saturated reds are clipped by the monitor before they can be displayed. This is a common problem when ProPhotoRGB is used with modern digital cameras.
A gamut mapping application such as ColorThink or GamutVision can show a delta E demonstrating the amount by which the colors are out of gamut. This pseudocolor image from GamutVision demonstrates the delta E according to the key immediately below the image. There is no reason that Photoshop could not have a similar display. However, the situation is less dire than it might first appear, since the weak link in the imaging chain is often the printer. In this case, soft proofing works well with my Epson photo printer, but if wider gamut printers were to become available, soft proofing could be limited by the monitor. In this situation, the out of gamut display in Photoshop could alert the photographer to this situation, and further investigation with a gamut mapping application could be carried out.