The condition provided here is off a bit.
${Select[${Target.ConColor},Grey]} the closing square bracket being off a little bit.
But with there only being one thing being compared, could just as easily do
${Target.ConColor.Equal[Grey]}
Using a different spell based on mob health:
Set two DPS options. One for the normal spell you use, and set the condition to stop casting once it's lower health than you want it to be. Then setup a second spell, with lower level/mana requirement and have it's condition oppose the first.
DPS1=BigNuke|99|Cond1
DPS2=SmallNuke|99|Cond2
Cond1=${Target.PctHPs} > 40 && ${Target.ConColor.NotEqual[Grey]}
Cond2=${Target.PctHPs} <=40 || ${Target.ConColor.Equal[Grey]}
So for Cond1 we want to use the BigNuke if the mob has more than 40% health, and they are not a grey mob.
For Cond2 we want to use the SmallNuke if the mob has less than or equal to 40% health OR they are a grey mob.