Most footage wont need to have it's colors adjusted, because it probably already looks fine. In some cases though, especially with older anime, we can really make it look a lot nicer by adjusting the colors a little bit. With the old version of this guide, people were blindly just copying and pasting color adjustment settings for some reason, often making their footage look horrible. If you don't know how to properly adjust the colors (if they even need to be adjusted, which they usually don't), then you shouldn't even mess with them.
Anothing important thing that you need to note--and this applies to all filters, not just color adjustment filters--you must make sure your settings work on the entire video, not just one or two frames that you happened to look at.
Color adjustment sample
Spoiler :
Ok, so as you can see from the images, this original source was rather dull, and it had a very strong pink/red tint. Panda should not be pink! To tweak the colors, I started with ColorYUV(). As you should know, in a YUV colorspace, the color of an image is stored in the U and V channels, so by manipulating these two channels, we can change the overall color tint of the image.
The two main parameters of ColorYUV that you need to be aware of are gain_u and gain_v. By setting gain_u to a positive value, it will make your video more blue, and at negative values it makes your video more yellow. Positive values for gain_v, on the other hand, will make the video more red, and negative values more green.
For the particular source pictures above, I played around with these values until I arrived at what looked most correct. I ended up using a gain_v value of -25 in order to remove the strong red tint, then I used a gain_u value of 5 to give it just a slight bit more blue. The settings I arrived at were ColorYUV(gain_u=5, gain_v=-25).
After getting the color tint correct, I thought that the colors looked a little faded, so I needed to increase the saturation a good bit. For increasing saturation, you can use the Tweak() command. By inserting the "sat" parameter into it, we can adjust the saturation. 1.0 is the standard saturation, 2.0 means twice as saturated, 0 means completely desaturated. For this source, I used Tweak(sat=1.2). Most anime should have perfectly fine saturation, although some older titles tend to be faded and can benefit from a slight increse. Tweak also has some other parameters for things like brightness and contrast, but I strongly recommend against using these. Instead, you should use levels.
Finally, I thought the video was a bit dark, so it needed to be brightened up a little. For this, I used the Levels() command. There are probably hundreds of tutorials on the internet about using Levels, so I'm not even going to try and attempt to explain it here. If you don't know how levels works, just google for "levels tutorial" or "photoshop levels tutorial". I doubt that most sources would need any adjustments to the levels, but in order to use this filter it is suggested to use the Levels virtualdub filter in order to obtain the correct values to insert into the filter. More information on the avisynth levels filter can be found in the documentation.