You can apply black and white, gradient map, invert, vibrance etc to a photo etc and you can change the existing colors of a photo in a dramatic way. There is no opacity or blend in the feature itself, it applies and you get the sepia or bright garish design etc but how to lessen it. How can you fade Photoshop image adjustments
Edit menu and fade
You can set the opacity to 0% and see the original photo, set it to 100% and get the full on effect. Set it to 50% and you get that lovely in-between look though with some like invert you just end up with a solid gray. You can vary it. You can also use blending modes such as darken, pin light, linear light, lighten etc and again, this really depends on what you want to achieve with the effect. Try them out and see what works best for you.
You can always go back to the edit menu and re-use it as it is live until the time you apply another effect or filter or change a selection etc.
You may not wish to apply the effect such as black and white, vibrance to all of the photo etc. You can use the selection such as a rectangular marquee tool. Apply the effect and the result only be applied to that area and then go to the edit menu and fade the effect. If you deselect etc then that option will be lost as it will not know what to apply the inversion to.
You can also apply the effect. Set the history state via the history panel to the original photo and select the history brush and apply the original back on top of the black and white / vibrance etc effect. You can vary the brush size as well as select any of the presets to use as well as vary the blending mode for the application