Black and white image on TV when using an RGB SCART cable
A black and white or grey scale image when using an RGB SCART cable is usually caused by region mis-match when the television or up-scaling device (SCART to HDMI converter) is not in RGB mode when the SCART cable is wired for "sync over composite video". Region mis-match is when you are using for example a 60Hz console on a 50Hz TV, or vice versa.
A black and white picture can also occur when using a "sync over luma" RGB SCART cable when the television or up-scaling device is not in RGB mode. And this has nothing to do with region mis-match.
A television or up-scaling device usually requires a trigger signal, also known as RGB selection up (https://en.wikipedia.org/wiki/SCART) which is provided by the RGB SCART cable and this tells the device to switch from composite video mode to RGB mode. This is usually in the form of a 1-3 volts signal from SCART pin 16. If this signal is not present the television or up-scaling device will remain in composite video mode (CVBS) and will result in a black and white / grey scale image. This signal can be missing for a number of reasons, i.e. incorrectly wired SCART cable, faulty SCART cable or use of cheap crappy SCART switch.
Also make sure that you have plugged your SCART cable into an RGB enabled SCART socket. As some televisions with more than one SCART input usually only accept RGB on the first SCART input, and the 2nd and 3rd SCART sockets maybe for composite video, or s-video only.
Finally, some televisions require you to select RGB mode as the input for the SCART, because the SCART can transmit other video formats besides RGB, including composite video, s-video and component video. So you may need to select the correct input using the remote control. This is usually labelled as "input select" on the remote control, and you need to keep pressing this button until you get a colour image.