This might have something to do with it. CS5 was released in April 2010, OS X 10.10 was released June 2014. Even with the updates to CS5 you're asking a lot for it to run in an OS that was released four years after AE was released. Make sure your CS5 is running the latest update. If that doesn't fix it you'll either have to downgrade your OS or upgrade AE.
Some people solved this issue by changing the preferences for use system color picker. Otherwise, what Rick said.
Thank you Rick, much appreciated, so would you know which AE will run on my system,? by the way my version of AE was a 30 day trial version of CS5 which I wanted to check out before purchasing.
my version of AE was a 30 day trial version of CS5 which I wanted to check out before purchasing.
You, um, you can't purchase CS5. CS6 is the oldest version of After Effects you can still purchase, and you need to actually call Adobe on the phone for that. CS5 is version 10. Version 10.5 was released, then version 11, then version 12, then 12.1, then 12.2, then 13, then 13.1, then 13.2, then 13.5, then 13.6, then 13.7 so...CS5 or CS6 are really far behind.
Technically, CS6 (as long as it's been updated to 11.0.4), will work on El Capitan, but it will not have motion sketch and there will be a weird RAM preview bug as well. If you subscribe to the Creative Cloud, you can get all versions of AE from CS6 through to the very latest (CC 2015 updated to version 13.7).
Thank you Szalam
So If I get CC2015 do you think that will work on my system, I am running Mac pro 10.10.5 yosimite 6 core. I find it difficult to get a answer from my usual sources
CC 2015 updated to version 13.6.1 seemed to work quite well on 10.10.5 from the forum posts I saw. Version 13.7 (while still too new to be sure) SEEMS to be behaving itself too.
If you're having issues with the brand new previewing architecture begun in CC 2015, CC 2014 (updated to 13.2) worked pretty well too.
Remember, you can test the newest version for 30 days to be sure before you purchase.