-First, put down the character you want to custom color their clothing. Make sure to pose them in advance.



-Duplicate your character, so the new one layers directly on top of the original. In this example, I moved the duplicated character to the side for better visualization.



-Turn off the character outline (Click 3D Outline Setting and set Weigh to 0)



-Remove the hair and put on a blank face. If your character comes with hair accessories that can't be removed via the "Change Item" menu, you'll have to create a duplicate character and remove hair manually.




-Once done, the duplicated character should look like this (on the right). We're ready to custom-color her.



-Right click -> Change Skin Color, set to white (255,255,255). You could select another color that doesn't clash with the default color of the clothing, just write down this number.



-Next, Right Click -> Change Color. Check Make specified color transparent, insert 255,255,255 to the RGB box. Increase the "Expansion" value to 96 or similar, and make sure to check "This color" on "Transparent Area".
-At this point, you have a duplicated model with transparent skin/hair on top of the original character, with only their clothes visible. Just change the color using the Hue / Brightness / Saturation / Contrast / Increase or decrease Color in the top forms. In this example, I made a simple hue change to make the uniform green.



-And you're all set. If you move the duplicated character away by accident, use the Numeric Setting to move it back to the original coordinate (X-Y value)



This trick allows you to change character's clothes as a whole, but you won't be able to change specific part of the clothes such as accessories, decoration image on the shirt, etc... Be warned though, this will double the amount of 3D characters you need to put on a page, so don't make too many panels with them.

Thanks for reading.