Minecraft java: How to summon a custom splash potion

Solution 1:

summon potion ~ ~ ~ {Item:{id:"minecraft:splash_potion",Count:1b,tag:{Potion:"minecraft:swiftness"}}}

This will summon a splash potion of swiftness, and gives the swiftness effect.

summon potion ~ ~ ~ {Item:{id:"minecraft:splash_potion",Count:1b,tag:{Potion:"minecraft:swiftness",CustomPotionColor:16711680,CustomPotionEffects:[{Id:3b,Amplifier:5b,Duration:200},{Id:5b,Amplifier:5b,Duration:200}]}}}

This will summon a splash potion of swiftness, that also grants haste 5 and strength 5. CustomPotionColor:16711680 changes the potions display color, and is formatted in decimal form.

I would recommend using MCStacker to generate potions, as NBT can get very complicated in potions.