Choose any color semantic-ui provide for example:
<Form>
<Form.Input label="Email" type="email" />
<Form.Input label="Password" type="password" />
<Button color="teal" type="submit">
Sign In
</Button>
</Form>
Your button appears like:

You can add inverted props inside Button component that react semantic-ui provide
<Form>
<Form.Input label="Email" type="email" />
<Form.Input label="Password" type="password" />
<Button inverted color="teal" type="submit">
Sign In
</Button>
</Form>
your component appears like:

On hover returns to basic style opposite of inverted
styled components usage with react semantic ui
I recommended you to use styled-components in order to override semantic-ui component style
import { Tab, Form, Button, Grid, Icon } from "semantic-ui-react";
import styled from "styled-components";
const Mybutton = styled(Button)`
&:hover {
color: #fff !important;
}
`;
Next use your new styled component instead of semantic-ui
<Mybutton inverted color="teal" type="submit">
Sign In
</Mybutton>