Sheesh, don’t get me started on the the Mac materials interface, it really could use some improvement.
However, focusing on your question I see the same behavior as you but this is as it has been for some time now. I have 18pro and 16pro both running on Mojave (10.14.5) and the color “pallet” icon is not present in either of these versions. It is present if I open the color picker in another application as you indicated, but not in Sketchup. What was your previous Sketchup version? Like-wise the color swatches automatically stack up left to right in both 18 and 16 in Mojave, I cannot have “empty spaces”. I believe this part is Mac behavior as they are responsible for the shared color picker UI.
The intended workflow for making and saving custom collections of colors and materials is through the Materials icon which is a brick.
Once in the materials pane you have the option near at the bottom of the panel under List, to choose New. You will be prompted to give your list a name, choose your name carefully as there is not a function to rename a list from within Sketchup. You can navigate to the custom material folders on your hard drive under library-application support-sketchup18-sketchup-materilas to rename a folder on the hard drive which will be then be reflected in your list on the next startup of Sketchup as Sketchup only reads the list on startup, not actively. The materials collections list only sorts alphabetically with no organizing options, so I name all my custom material collection starting with “z” then a custom name so they get grouped together at the bottom of the list and are not lost among the standard collections.
Now, this is a tricky part, once you have a new List created, the name of the new list will be appear in the selection at the top of the panel indicating you are in that folder, however you are not. The swatches present in the panel are from whatever list you had previously up before making a new list. You must use the drop down menu to navigate away from the new list once to any other list then back to the new one it to actually get into the new folder. This is a bug.
Once in the new folder you can only add new materials / colors by dragging them up from the swatches below, or from the current selected material which is the larger one on the left. You can also drag in image files (png, jpg, and the like) from your desktop and drop them into the current folder space in the pallet to make a new texture using that image, or right click in the empty space of the pallet and select new texture which does the same thing. Unfortunately there is no “split screen” option for the pallet like in the Windows version of Skethup so there is not an easy way to quickly move materials from one saved list to another. There are kludgy work arounds I won’t go into here.
When importing a new texture… a couple of quirks.
You will be prompted to name your new texture, be careful with your name. There is no reliable way to change the name of a saved material once made. You can double click a material in the folder area and chose Edit, this will open a few options at the bottom, one of which will be the name of the color/material, you can type in a new name. However this exposes a bug in the Mac materials management, the new name you type will only persist for that session of sketchup. On restart of Sketchup, odd things happen. best case is that the name simply reverts to the old name, which it does in about 70% of case, sometimes however the material will be mysteriously duplicated in the folder, so you will have one of the old name and one of the new name. If you renamed 20 materials you have 20 duplicates. Your experience may vary but In general I have very little luck renaming saved materials.
When importing a new material you are prompted to set the dimensions of the material. Keep in mind that althought the height and width are set as linked by default in this window, they are not, changing one value does not actively change the other. If you type in a new number for the upper value of width, the bottom number which shoudl be linked as an aspect ratio does not change and the OK button remains highlighted. This means that if you change one value and press return on the keyboard, the new file instantly gets imported with only the change you made to one of the values. You end up with textures 20 feet wide by 3 inches tall because the linked value failed to update. This is a bug. You must mouse click once into the second linked value to get it to update before pressing return.
Finally be aware that there is not a functioning way to remove a list from within Sketchup. There is a Remove option under the List menu in the color picker, if chosen there is even a warning window that pops up to ask if you are sure you want to remove the list. This action does nothing. Any collection removed this way is only gone from the master list for that session of Sketchup and will be returned to the list of collections on the next restart of Sketchup. Removing a list from within Sketchup does not work, you must navigate to the folder location on the hard drive and trash it.
This is not intended to be a comprehensive list of the “quirks” of the Materils picker in the Mac version, there are others. It’s just something of a road map for navigating the basics of how to make it work. I have not seen this information presented anywhere else and much of it is the result of hours of experimenting on my part and helpful insights from others on the forum. Perhaps it might save others a bit of frustration.