Early models of the CDTV had OCS chips and later ones could have a mix of OCS and ECS, and some late models could have the full ECS chipset. It could also be upgraded to ECS by the user. The CDTV II that you link to was never released though so that's probably not what you have unless you own a prototype.
But even if you have the full ECS chipset it didn’t increase the amount of colors in the PAL/NTSC compatible low and high resolution modes. In high resolution, with or without interlace, 16 colors is the maximum supported.
To get more colors you need to run in low res, where up to 32 colors are possible. In low res there’s also the special EHB (extra half bright) that adds another half as bright shade to all of the 32 colors chosen, for a total of 64. Or the other special mode, HAM (Hold-And-Modify), that gives 4096 colors with some quite noticeable limitations.
The ECS chipset also added some new flicker free VGA compatible screen modes like Multiscan in 640x480 at 60 Hz and Euro 72 with 640x400 at 72 Hz (all of them could be doubled in height with interlace). They were only available with up to 4 colors on ECS.
The reason for all of these limitations are basically limited bandwidth, and running in higher resolutions with max colors can also feel quite slow.
All of this is quite easy to check, the Screen Mode prefs won’t let you choose more colors than the chipset supports for that mode.
The information in the corner of your Workbench screen isn’t a standard feature in the OS, it’s probably added by Scalos. I don’t know if it’s correct or not but you can check which chipset your Amiga uses with a tool like WhichAmiga.