What I Think.
Season 1: Animated on cels giving the season a natural, underwater look, and the colors were dull consistently.
First S1 Episode:
http://i.imgur.com/OCtADcm.jpg
Last S1 Episode:
http://i.imgur.com/zNKyaUF.jpg
Season 2: First season to be digitally animated, making the colors give a slightly warm tone, but the early season 2 episodes were somewhat pale (Example: Something Smells/Bossy Boots)I assume they were just getting used to the color palette.
First S2 Episode:
http://i.imgur.com/rukq54d.jpg
Last S2 Episode:
http://i.imgur.com/HToCVl3.jpg
Season 3: The season had darker colors, but more vibrant than season 2. The colors are alot warmer than in season 2, and they look less natural for me, especially late season 3, but that's just me.
First S3 Episode:
http://i.imgur.com/OzgJi7u.jpg
Last S3 Episode:
http://i.imgur.com/wCrIPEa.jpg
I'm skipping the movies because they aren't technically a part of the show.
Season 4: The seasons color palette changed, DRAMATICALLY. The colors turned cooler, and isn't as dark as season 3, and Mr.Krabs' shell is all of a sudden a orange/red/pink mix. I've heard rumors that they used, 3-D computer coloring/inking, but I don't sense any 3-Dimensionalism.
First S4 Episode:
http://i.imgur.com/yLS5zEe.jpg
Last S4 Episode:
http://i.imgur.com/CI8lRnv.jpg
Season 5: Again nothing too different with the colors, but in the season they switched to tablets for the storyboards near the end.
First S5 Episode:
http://i.imgur.com/Y7X08Zk.jpg
Last S5 Episode:
http://i.imgur.com/zqm5Q69.jpg
Season 6: This season has brighter colors than season 5, and that's about it.
First S6 Episode:
http://i.imgur.com/p41z0e7.jpg
Last S6 Episode:
http://i.imgur.com/MTT3Cxp.jpg
Season 7: Again no major changes, the same palette.
First S7 Episode:
http://i.imgur.com/KoCTj3B.jpg
Last S7 Episode:
http://i.imgur.com/7aMwSsW.jpg
Season 8: Now, this season has some VERY cool colors. they are not very vibrant, and honestly, it looks so bland. The colors feel so dead to me.
First S8 Episode:
http://i.imgur.com/CAYk6Gb.jpg
Last S8 Episode:
http://i.imgur.com/zpO8BbV.jpg
Season 9: This season was broadcast in HD, so the colors of course will get brighter, but it isn't to bad, yes, I know, season 9A didn't do good with coloring, but 9B fixed it, and the colors are vibrant enough.
First S9 Episode:
http://i.imgur.com/ypyMCjF.jpg
Last S9 Episode:
http://i.imgur.com/QfF9U9q.jpg
Season 10: This season has alot in common with 9B, so not much to say.
First S10 Episode:
http://i.imgur.com/RHsPbdr.jpg
Last S10 Episode:(Unaired)
Season 11: Is it just me, or or are the colors dimmed in this season? Maybe I'm colorblind, but I feel like that is the case.
First S11 Episode:
http://i.imgur.com/Q1jjNrM.jpg
Last S11 Episode:(Unaired)
BTW, I used waifu2x for remastering the images.