Is it possible to fill a string or number with two colors?

There are at least two obvious ways to do this.

1) use a clipping rectangle to draw the top half in one color, and then update the clipping rectangle to draw the bottom in another.
2) use a custom font that defines the font to look like cutouts (the glyph is transparent and the outside is opaque). Draw the colors you want as rectangles, and then overlay the text stencil. (the NoFrills technique).

The following code should work. I haven't actually tried to compile or run it, but it is theoretically pretty close to what you'd need.

function onUpdate(dc) {

    dc.setColor(Graphics.COLOR_GREEN, Graphics.COLOR_BLACK);
    dc.clear();

    // these could be member variables and you could change the values at
    // runtime as you see fit
    var text = "Hi!";
    var percentOfTextThatIsGreen = 0.30f;

    var font = Graphics.FONT_LARGE;
    var dims = dc.getTextDimensions(text, font);

    var width = dc.getWidth();
    var height = dc.getHeight();

    var centerx = width / 2;
    var centery = height / 2;

    // assumes that font glyphs fill the entire fontHeight. this is not true for
    // all built-in font glyphs, so you might want to use a custom font for this

    var topOfGreenText = centery - (dims[1] / 2);
    var botOfGreenText = topOfGreenText + (dims[1] * percentOfTextThatIsGreen);

    var topOfRedText = botOfGreenText;
    var botOfRedText = topOfGreenText + dims[1];

    // there may on off-by-one error here because i'm trying to center the text
    // at (centerx, centery).. the divide by 2 will truncate, possibly clipping
    // one pixel on the left.
    var leftOfText = centerx - dims[0] / 2;
    var rightOfText = leftOfText + dims[0];

    // draw the top in green
    dc.setColor(Graphics.COLOR_RED, Graphics.COLOR_TRANSPARENT);
    dc.setClip(leftOfText, topOfGreenText, rightOfText, botOfGreenText);
    dc.drawText(centerx, centery, font, "Hi", Graphics.TEXT_JUSTIFY_CENTER | Graphics.TEXT_JUSTIFY_VCENTER);

    // draw the bottom in red
    dc.setColor(Graphics.COLOR_RED, Graphics.COLOR_TRANSPARENT);
    dc.setClip(leftOfText, topOfRedText, rightOfText, botOfRedText);
    dc.drawText(centerx, centery, font, "Hi", Graphics.TEXT_JUSTIFY_CENTER | Graphics.TEXT_JUSTIFY_VCENTER);

    // reset the clipping area
    dc.clearClip();
}