I've got some footage that i'm trying to match with another camera as best as possible. The footage i need to correct was shot with a Go Pro Hero 3 with no red filter below 40 feet, as a result the footage is very blue.
What is the best way to bring the colors to normal in after effects?
I'd duplicate the footage, apply Color Correction > Channel Mixer to the top layer and Color Correction > Levels to the bottom.
Then on the bottom layer reduce the Red output white to 0 in Levels (removing the red channel from the source).
Now rebuild the Red channel using the Channel Mixer on the top layer by setting the Red-Red to around 25, the Red-Green to around 135 and the Red-Blue to something around -70. All other properties of the Channel Mixer should be 0.
What this will result in is a Red channel that is a weighted blend of the Red, Green and Blue channels. Since there isn't much information in the source Red channel we're weighting that lightly (Red-Red 25), this will keep some of the original red information. Since the Green channel has a lot of information (and it's not the Blue channel) we weight that heavily (Red-Green 135). Because we don't want the blue water to get red we also subtract information from the Red channel where the source Blue channel was intense (Red-Blue -70).
You should be aware that these 3 numbers add up to 100. If they add up to more than 100 then the Red channel may look too bright, if they add up to less than 100 then you're not adding much Red. However, in your case (since there is so little Red in the source footage) you may benefit from increasing the Red-Red contribution in the Channel Mixer.
Finally set the top layer to the 'Add' blend mode to add the new Red channel back into the source Green and Blue channels.
Here's a sample AEP where I've also added a couple of final adjustments for a nice look: