It has come to my attention that someone has beat me to it while writing my answer, but I am proud of my work so to compensate for wasting stackexchange server space I will make my answer very colorful.
Complete Explanation (unspoilered) (why did you scroll down here if you didn't want the answer smh):
Single cell groups must have a 1, obviously.
Any orange cannot be 2, otherwise a 2 cell group (blue) cannot have a 2.
If a yellow was 2, then there would be two 2s on the same row in blue. Contradiction by rules.
Therefore, green is 2.
You know normal sudoku, where you see if a number kinda covers everything in a 3x3 square except one square, then you know what number that square is? We can kinda abuse that everywhere in a similar manner, like with the 1 and 2 I put in the 2 cell group in the center right there.
We abuse this strategy to get:
Clearly, only the reds can contain the 3 in their 5 cell region. By ghost, neither orange can contain 3. Thus, yellow is 3.
We have a chain of trivial deductions:
Trivial deductions in the top left. Because of numbers in red, the cell to their left must be 4.
Bunch of deductions in the bottom right, following the rainbow.
Follow the rainbow. Since there is a 4 in green, there is no 4 in blue, therefore purple is 4.
Because of red, orange is 3. Then we can follow the rainbow of deduction!
Rainbows. Just rainbows.
So what's your favorite color? Mine is green!
But you see, that's mostly just white, so for the sake of color: