Lighting Problems
In the testchamb_a_05.vmf example map, there are some entities near the entrance that you may want to copy and paste into your map. There's a color_correction and an env_tonemap_controler, and also a shadow_control if I remember correctly. These entities tweak some colour and lighting settings for Portal.
If you already have a color_correction, make sure it has the right lookup table set, otherwise it won't do anything. (Somehow I feel I'm again telling you something you already know, but hey... 
)
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
I was aware of the usage of bluish lights used in the testchambers, and I don't think I've left a light at its default value in any of the maps. I use a gray color, because it's pretty close to the blue used in Valve's maps; I also like the way it looks. Thank you for the advice, though--I'll try modifying all the gray lights to the shade of blue used in testchamber_a_05 and see how it turns out.
Thanks for the heads up. I already put color_corrections and shadow_controls at the beginning of the maps. I missed the env_tonemap_controllers, though, and those have been added. I checked just to make sure, and all the values for the trio are identical to those used in Valve's maps (or at least testchamber_a_05), and the lookup table is indeed testchamber.raw.
On a side note, while trying to find the VDC article containing the light values mentioned by Mek (couldn't find it--no big deal, but could you point me in the right direction?), I read up on pointfiles. Heeding the advice, I have now gone through and corrected any leaks; there were no actual gaps, just accidental func_details. I don't know if it will make a lighting difference (as there was only one in each map), but, hey, the fewer errors the better!
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You definitely want your maps to be leak-free. The difference in lighting will be pretty obvious, and a leaked map also runs a lot slower, because none of the engine optimisations work. (Each part of the map is always drawn, even when it's hidden behind e.g. a wall.)
Be sure to re-compile and check the point file again when you have fixed the leak. It will only report one leak at a time, but your map may contain more than one. Leaks in maps are a lot like mosquitoes. You swat one, turn off the light, go back to bed, then the next one turns up... 
You can also easily check if a map has leaks by using "noclip" in-game and move outside the map. You should be able to see the inside of the rooms, like a diorama. If you see all the outer walls of your map, you still have a leak somewhere.
As for the lights, the only point light I use is for the observation rooms. Valve uses the colour "183 204 218 800" for these.
For the light strips in the ceiling: LIGHTS/LIGHT_RECESSEDCOOL002
For block hole lights: LIGHTS/WHITE001.
If you have no leaks, the light from these textures will "bounce" off other surfaces and cause a pretty realistic lighting, so you don't need to manually place fill lights.
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
I discovered the bit about only displaying one leak at a time after I posted--sorry. All the leaks are now fixed. I just tested the maps for the first time after the leak fixing, by the way, and you're absolutely right--I was blown away by the difference in lighting! Thanks for the advice.
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Not for me. You just map neatly and you will be ok.
But yeah, from the image, looked like you had a leak, but I thought you fixed it already so I did not say anything.
The more you give, the less appreciated it will be received.
I could't find it either now, sorry... but you can look up the color values in the sample test chamber vmf anyway..
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Does mapping neatly get rid of mosquitoes? That would be nice 
But yes, this experience is from the days when I just started out mapping and was placing brush entity centre handles outside the map, func_detail outer walls etc.
Once you get better at mapping, this problem pretty much goes away.
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
I was aware of the usage of bluish lights used in the testchambers, and I don't think I've left a light at its default value in any of the maps. I use a gray color, because it's pretty close to the blue used in Valve's maps; I also like the way it looks. Thank you for the advice, though--I'll try modifying all the gray lights to the shade of blue used in testchamber_a_05 and see how it turns out.
Done! I tested it out using the bluish color; it works much better. I'll change the lights to the new color throughout the maps.
Now with 100% more inline comments!