If you want to provide some basic colors, semantic ui already has the classes for you. You can add the classes red ,orange, yellow, olive, green, teal, blue, violet, purple, pink, brown, grey, black to the button.
<link href="https://cdnjs.cloudflare.com/ajax/libs/semantic-ui/2.3.3/components/button.css" rel="stylesheet"/>
<button class="ui red button">Red</button>
You can also add the class basic to make only the button outline colored.
<link href="https://cdnjs.cloudflare.com/ajax/libs/semantic-ui/2.3.3/components/button.css" rel="stylesheet"/>
<button class="ui red basic button">Red</button>
If you want to provide a custom color this question already has some answers.
In your question's case, it's more difficult. You want only the outline and text-color changed. If you inspect the button, you'll they use the property !important for color and box-shadow (yes, they're using box-shadow property instead of border property to create a border). So you too will have to use !important.
#blah input.ui.blue.big.basic.button {
box-shadow:0px 0px 0px 1px #8f3f9f inset !important;
color: #8f3f9f !important;
}
<link href="https://cdnjs.cloudflare.com/ajax/libs/semantic-ui/2.3.3/components/button.css" rel="stylesheet"/>
<div id="blah">
<input class="ui blue big basic button" type="submit">
</div>
The idea is that you have to make it more specific than the semantic UI properties.