"striped" colors on Android

I had a tablet (Alcatel U3A_7_WIFI running Android Go) working alright. In order to root it, first I made (what I thought was) a full backup with mtkclient : python mtk.py rl ./backup After the rooting went wrong, I tried to flash a stock ROM (with SP Flash tool). Found many variants for that model and they all work perfectly while doing black and white output. But as soon as colors are involved I get this stripe pattern (and bluish colors): The battery indicator shown when charging, has the same color problem, so I think it has to be something happening at a very early stage. Does anybody know how to solve it? Or what could be wrong? I tried flashing individual partitions with mtkclient from the backup I had (no way to flash the whole thing back, as far as I know). But half of the time this bricks the tablet (just BROM mode) and the other half nothing changes. Colors are still the same, bluish and striped.