There are only two Denise types: OCS and ECS - it doesn't care whether NTSC or PAL (see
BBoAH). Horizontal scan rate is exactly the same (well nearly, the crystal differs) and vertical scan rate is taken care of by Agnus.
When you're working with the CVBS signal (one signal only, mostly RCA jacks) PAL and NTSC do completely different color encoding. The RGB -> NTSC converter probably outputs NTSC style color, regardless of frequencies,
but the TV tuner expects PAL colors when fed with 50 Hz pictures, that's why it's b/w. Sorry, apart from using a different/PAL compatible RGB->video converter there's no way around it.
I'm having the same type of problem when trying to digitize NTSC videos here in Germany: I've got an 'NTSC playback' VCR, that
recodes the tape's NTSC to PAL and outputs with 60 Hz (='PAL 60'). All video cards I've tried expect an NTSC color code and won't see the PAL color. The only way out is a real NTSC VCR.
PS: I've bought a genlock at eBay to convert Amy's RGB to PAL SVideo - much better picture than CVBS and cheap. :-)