Coyote's line of high fire underglazes are all designed to be bright and true from cone six to cone ten! Available in a perfect 4oz bottle.
Out of stock?
Turquoise blue underglaze by Amaco: https://amaranthstoneware.ca/collections/velvet-underglaze/products/turquoise-blue-velvet-underglaze-by-amaco
Baby Blue Underglaze by Amaco: https://amaranthstoneware.ca/collections/velvet-underglaze/products/baby-blue-velvet-underglaze-by-amaco
Or Sky blue from coyote which you can mix with white! https://amaranthstoneware.ca/collections/underglazes/products/sky-blue-underglaze