WLED JSON Change colour of part of my strip

I’m new to WLED and have a strip of LEDs currently in 3 segments (but it could be just 1) I can send json requests via curl and get the response. I want to have an effect running and then change a specified number of LEDs to white, but leave the rest running the effect. Then later switch off the white and return to all LEDS running the effect. I can’t work it out. I used the “seg” “i” commands but got only the white LEDs, the rest went off. Anyone help me please?