You can either use the existing DateTime field with the initial value something like..
TODAY()&" "&
SWITCH([Color],
โYellowโ,โ09:00:00โ,
โBlueโ,โ10:00:00โ,
โ12:00:00โ
)
If you want to select the date individually, add an additional date column and then use that date with your existing DateTime column in a same way.