I was on my friend's server, which he wanted me to make a custom potion for him that doesn't give him the effects, which also has the color of the harming potion. So I did the following command:

/give @p minecraft:splash_potion 1 0 {CustomPotionEffects:[{Id:7,Amplifier:0,Duration:0}]}

which gives you a splash harming potion. So when I tested it out, for some reason, I still received the harming potion effect. Same thing happens with splash potion of healing.

Video link: https://youtu.be/TiY_0HK1I7Y

Code analysis by marcono1234 in this comment