How do I bring up color gradually on a slide?

Question

I am new to Keynote and to making presentations. I have just about zero familiarity with Keynote tricks of the trade or lingo, so please keep that in mind if you answer my question.
I recently got as an e-mail attachment a PowerPoint presentation that when opened and played as a slideshow in Keynote gradually brings each slide up from no color (i.e., black and white) to full color, then instantly moves to the next slide without a transition.
How do I create a slideshow in Keynote to do that?
The PowerPoint slideshow I received is basically a series of color slides of nighttime cityscapes from around the world that look beautiful in Keynote as the color is brought gradually up from zero, over the course of about 10 seconds per slide. When I play the show in an ancient copy of PowerPoint (v. 10.1) that I have lying around, I have to click once for each slide to change it instantly from B&W to color, then again to move instantly to the next slide. The instructions that appear superimposed over the slides indicate that this is exactly how the show was designed to be played, so I assume what Keynote is doing (playing the whole show straight through without my needing to click the mouse, introducing the color gradually for each slide) is an "extra" Keynote is adding.
What I'm asking here is how to introduce the color gradually for each slide in the presentations I make. Also, can I do that just for an individual photo or graphic on a slide that has text elements and a background, as with a photo in the "Photo - Horizontal" layout. If so, how would I get this effect?
Thanks in advance for your help.

Answer 1

Kyn,
Thanks, that answered my question.
To add to what you said, I figured out the same basic thing on my own (though I didn't think of using a Build dissolve and was using a Transition dissolve). I discovered the easiest way to get the B&W version of the image was to make a slide with the image in color and then duplicate it. Selecting the image _in the first slide of the pair_, I used View menu —> Show Adjust Image to bring up the Adjustment controls and dragged the Saturation control all the way leftward to remove the color. Then for the B&W version of the slide, I set up the Transition in the Slide Inspector to use a 3.00 second Dissolve, with the Transition started automatically. For the color version, I decided to use a Shimmer transition with a 1.50 sec. duration, starting automatically with a 5.0 sec. delay prior to the next pair of slides.
All that is fairly easy to do, if hard to describe, and it produces an automatic slideshow in which all the photos move gradually from having no color to having full color, remain on the screen briefly, then do a Shimmer transition to the next photo.
Thanks for your help in confirming how this can be done!