Hi, I've looked but maybe I'm not using the right terms (I'm a French speaker). So sorry if this has already been discussed.
Is there a way to gradually increase or decrease the intensity of a light and/or its color? For example, to simulate a sunrise/sunset. Thanks for “enlightening” me 
Yes. In rule machine, you want the command Fade Dimmer over Time (it's under Set Dimmers and Bulbs). Basically, you set the new level and put in the fade time parameters. This will raise or dim your bulbs from the current level to the new level or the selected time.
Color is a lot trickier. In rule machine, it generally involves lots of repeats of hue/sat/level to gradually raise (or lower) both the level and color at the same time.
My experience with dimming lights over time is that the progression is not analog but more digital. Like 1% steps until we get to the minimum dim level for the device then it turns off. I found that regular dimmable LED bulbs dimmed through a dimmer switch get much dimmer than do Smart LED bulbs which seem to bottom out around 5 to 10%.
You probably are describing circadian lighting or adaptive lighting.
There are (or were) a couple community apps for this application if you search circadian lighting.
Download the Hubitat app
