The gradients were created with 15-16 blocks of distance for maximum biome blend. As such, they mix with biome transition gradients best when the Biome Blend setting is set to 15x15.

Adding a gradient:
1. Highlight current biome ID
2. Primary color is hills, secondary is default
3. Start at pixel height 108 and drag gradient to pixel height 92
4. Deselect
5. When finished, move all pixels down 64 and stretch bottom pixel up 64 blocks

A few biomes have special gradient heights:

Deserts and Badlands have gradients from y=88 to y=72.
Swamps and Mushroom Fields have gradients from 80 to 64
Taigas and groves have two gradients, one from 108 to 72, and another from 158 to 142.
All oceans have fog gradients (between normal and deep colors) from 53 to 38.

Here are the biomes with custom colors (not from Bedrock Edition):

Plains Hills (not its own biome, but high up in plains biomes)
Grove
Snowy Slopes
Frozen Peaks
Jagged Peaks
Stony Peaks
Dripstone Caves
Lush Caves
Deep Dark
Warped Forest
Crimson Forest
Soul Sand Valley
Basalt Deltas
End Highlands
End Midlands
Small End Islands
End Barrens

The default water color (used past the last biome IDs) is Ocean.