Lava is affected by the new "colored water by biome" feature in new snapshots.
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Say something silly, Laugh 'til it hurts, Take a risk, Sing out loud, Rock the boat, Shake things up, Flirt with disaster, Buy something frivolous, Color outside the lines, Cause a scene, Order dessert, Make waves, Get carried away, Have a great day!
It sounds like you are describing https://bugs.mojang.com/browse/MC-128233 "Lava is tinted incorrectly" (has pictures of "pumpkin poop"). This appears fixed in 18w16a, the very latest available release. 18w17a is due out tomorrow unless there are problems. Keep track of that bug link. If you are still having lava tint problems in 18w16a on, Mojang needs to know so they can fix it. ;-)