I also added a touch of Mars brown that did touch some of the aqua
blue. When the piece cooled all of the blue had turned a muddy fir
tree kind of green.
Brown is a composite of many colors, so it probably has some of the
same oxides as yellow and red, and so it reacted with the silver. I
have had reactions with the silver even though there are "safe"
colors under them. Often had red turn brown when there was clear
flux under it.
The other question relates to Thompson Acrylic Enamels.
The orange, red and yellow are really temperature sensitive. Try
firing at 1300.