Figure 1: Shall I compare thee to a summer's day? Thy color is more lovely and more linearly related to the pH of the solution. [Source]
The bug I'm working with ferments sugars for a living, so as it grows it makes its environment more acidic. This is really handy, because we have wonderful molecules like phenol red that change color according to pH. In this particular case, yellow means acidic and red/pink means basic, so as the bacteria grow, the ratio of yellowness to redness should tell us something about how long the cells have been growing and how many there are. Here's a paper by another group that used this method, although they did so much other (amazing!) stuff that you'd have a hard time finding the details (look at figure S4 on page 41). So it really does boil down to "just look at what color it is", except that if you need to be precise, you use a spectrophotometer instead of your eyes.
The trouble is that the yellow/red ratio doesn't actually tell you anything about the number of cells in your culture, unless you take the trouble to do it the hard way and the easy way at the same time and figure out how they correlate. This is called doing a growth curve, and it's something that has to be done for every different bug if you want to measure it by any method that's easier than counting spots in a dish the day after you wanted to know.
So... while I can't say I have been in lab for the past 24 hours (I took a couple hours off last night to go get food and a nap), I have been running this same experiment for the past 24 hours and taking samples more or less every hour. It's tiring in multiple ways. I could barely make myself come back from my dinner-and-nap break because I was in physical pain from being tired. Also, having to go do something for 15 minutes out of every hour is hell on your ability to get anything else done.
When I said I had blocked out the entirety of my IAP for this research position... I guess I really meant the entirety of my IAP. All hail science!