Weird palette with RGB modded SG1000 II

[kamillebidan]

I RGB-modded an SG1000 II (the one with the sega chip that integrates the TMS9918).

First I tried to use a THS7374 as amplifier. However, the image was very noisy (I put the amplifier outside the console, so I guess the wires were long enough to dirty up the signal).

I then tried the same circuit of the  Adapteur RVB, without the 12V generator part. The image is stunningly nice even with long cables(thanks Furrtek!). Furthermore, I could use the CSync pad as the sync signal (this was not possible with the THS7374, and even here or here they resorted to use the composite signal for synchronization.

I have, however, a weird problem. The palette shows much more "saturated" colors than I'd expect.
With reference to this palette, both dark red and light red are very close to what we'd say FF0000, and dark blue looks like 0000FF.  I'd say, the color are not the same of the Mark III (I tested it with the same monitor,and they're different), but they're sort of "half a way" between the classic SG-1000 palette and the Mark III one.

At the first of the links it is told that the signal is 1.4Vpp and should be halved. I tried to add the same halver (a voltage divider with two 1K resistors) but the result is only darker colors.

I do not think it's a problem of one channel being too different from the others (at the beginning I though the red channel was too strong) since white appears as white and gray is correctly rendered to gray as well.

If it may helps, the monitor I use is a Sony PVM20N2J.

Have you ever experienced this problem?
Do you have any clue on what I could try?