I placed this in my app.styl

$desired-color = purple
.q-if
.q-input-target-placeholder
color $desired-color !important
.q-input-target
&::-webkit-input-placeholder /* Chrome/Opera/Safari */
color $desired-color !important
&::-moz-placeholder
color $desired-color !important
&:-ms-input-placeholder
color $desired-color !important

May it helps