Emulating the OCS, ECS or AGA chipset is slow, instead, you can use the PC's graphics card instead to output graphics and use 256 or more colors.
Then, you need the Picasso 96 software which is not distributed with WinUAE. Version 1.31 or higher is recommended. After you installed the Picasso96 software, reboot the emulated Amiga. Now, you should be able to select the new modes - which are identiefied as uaegfx - from the ScreenModes program on the Amiga.
Make sure you utilize the latest rtg.library which is located in the Amiga Programs folder of your WinUAE installation. You have to copy that file into the following directory on your Workbench drive: libs:picasso96.
Start the program p96_uae_tweak from the Amiga Programs folder if you notice large slowdown when 16bit AHI is activated.