24 hours is not a long time, it may easily take 48 hours before enough cells are formed to give a significant lighter appearance. I've seen that many times. Yeast count can double every 90-120 minutes under optimal conditions, a stir plate or shaker at room temps and some nutrients (DAP/Urea/Magnesium/Zinc, etc.) help toward that goal.
It's important that yeast and wort are within 10°F of each other when pitching it. Within 5°F is even better. It prevents yeast shock and little mutants from forming.
Aren't you using a yeast calculator like
HomebrewDad's to estimate the cell count to pitch?
The slurry from a 2 liter starter contains about the appropriate amount of cells. 1.5-1.8 liter starters can be fine too.
If you "overbuild" your starters somewhat (making it a bit larger than you need) you can save some out to make a starter from for next time, and so on. But definitely not half, maybe 100-200 ml worth. I put that extra in one or two 8 oz (~240 ml) mason/jelly jars. Or pour some of the crashed slurry into a 4 oz jelly jar and add a few ounces of the saved supernatant to it for storage. 50-100 billion cells saved out will make a perfect starter for next time, even 3-6 months later.