For Laz 1.8.4, the color dialog behaves on Win10 like that for Laz-trunk which I reported above. The same with 1.8.4 on Windows 7. So, I don't know what's going on here...
Are there any special packages that you have installed? For a test, find out the directory in which your Lazarus profile is stored: "View" > "IDE Internals" > "About IDE". Exit Lazarus, rename the folder which had been mentioned in the "About IDE" window under "Primary config directory" by adding some character to the directory name. Restart Lazarus - it will not find its configuration (because you renamed the directory) and use default settings, without added components. Ignore the error messages. Recompile the IDE ("Tools" > "Build Lazarus with profile..."). When Lazarus has restarted try the ColorDialog issue again. Is it solved now? Please report the result.
To restore your old Lazarus, exit the IDE again, delete the newly created primary config directory and rename the old primary config directory back to its previous name. Restart Lazarus and rebuild the IDE again.