You should read Getting Started, please; it tells you how to tackle problems like this.
The general approach described there, applied to your problem.
1. create a docx containing the feature of interest. In this case, you get:
Using xml Syntax Highlighting
<mc:AlternateContent>
<mc:Choice Requires="wps">
<w:drawing>
<wp:anchor distT="0" distB="0" distL="114300" distR="114300" simplePos="0" relativeHeight="251659264" behindDoc="0" locked="0" layoutInCell="1" allowOverlap="1" wp14:editId="36B11C9B">
<wp:simplePos x="0" y="0"/>
<wp:positionH relativeFrom="column">
<wp:align>center
</wp:align>
</wp:positionH>
<wp:positionV relativeFrom="paragraph">
<wp:posOffset>0
</wp:posOffset>
</wp:positionV>
<wp:extent cx="2374265" cy="1403985"/>
<wp:effectExtent l="0" t="0" r="3175" b="5715"/>
<wp:wrapNone/>
<wp:docPr id="307" name="Text Box 2"/>
<wp:cNvGraphicFramePr>
<a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"/>
</wp:cNvGraphicFramePr>
<a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main">
<a:graphicData uri="http://schemas.microsoft.com/office/word/2010/wordprocessingShape">
<wps:wsp>
<wps:cNvSpPr txBox="1">
<a:spLocks noChangeArrowheads="1"/>
</wps:cNvSpPr>
<wps:spPr bwMode="auto">
<a:xfrm>
<a:off x="0" y="0"/>
<a:ext cx="2374265" cy="1403985"/>
</a:xfrm>
<a:prstGeom prst="rect">
<a:avLst/>
</a:prstGeom>
<a:solidFill>
<a:srgbClr val="FFFFFF"/>
</a:solidFill>
<a:ln w="9525">
<a:solidFill>
<a:srgbClr val="000000"/>
</a:solidFill>
<a:miter lim="800000"/>
<a:headEnd/>
<a:tailEnd/>
</a:ln>
</wps:spPr>
<wps:txbx>
<w:txbxContent>
<w:p w:rsidR="00F54378" w:rsidRDefault="00F54378">
<w:r>
<w:t>Here are the contents of the text box.
</w:t>
</w:r>
</w:p>
</w:txbxContent>
</wps:txbx>
<wps:bodyPr rot="0" vert="horz" wrap="square" lIns="91440" tIns="45720" rIns="91440" bIns="45720" anchor="t" anchorCtr="0">
<a:spAutoFit/>
</wps:bodyPr>
</wps:wsp>
</a:graphicData>
</a:graphic>
<wp14:sizeRelH relativeFrom="margin">
<wp14:pctWidth>40000
</wp14:pctWidth>
</wp14:sizeRelH>
<wp14:sizeRelV relativeFrom="margin">
<wp14:pctHeight>20000
</wp14:pctHeight>
</wp14:sizeRelV>
</wp:anchor>
</w:drawing>
</mc:Choice>
<mc:Fallback>
<w:pict>
<v:shapetype id="_x0000_t202" coordsize="21600,21600" o:spt="202" path="m,l,21600r21600,l21600,xe">
<v:stroke joinstyle="miter"/>
<v:path gradientshapeok="t" o:connecttype="rect"/>
</v:shapetype>
<v:shape id="Text Box 2" o:spid="_x0000_s1026" type="#_x0000_t202" style="position:absolute;margin-left:0;margin-top:0;width:186.95pt;height:110.55pt;z-index:251659264;visibility:visible;mso-wrap-style:square;mso-width-percent:400;mso-height-percent:200;mso-wrap-distance-left:9pt;mso-wrap-distance-top:0;mso-wrap-distance-right:9pt;mso-wrap-distance-bottom:0;mso-position-horizontal:center;mso-position-horizontal-relative:text;mso-position-vertical:absolute;mso-position-vertical-relative:text;mso-width-percent:400;mso-height-percent:200;mso-width-relative:margin;mso-height-relative:margin;v-text-anchor:top" o:gfxdata="UEsDBBQABgAIAAAAIQC2gziS/gAAAOEBAAATAAAAW0NvbnRlbnRfVHlwZXNdLnhtbJSRQU7DMBBF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">
<v:textbox style="mso-fit-shape-to-text:t">
<w:txbxContent>
<w:p w:rsidR="00F54378" w:rsidRDefault="00F54378">
<w:r>
<w:t>Here are the contents of the text box.
</w:t>
</w:r>
</w:p>
</w:txbxContent>
</v:textbox>
</v:shape>
</w:pict>
</mc:Fallback>
</mc:AlternateContent>
</w:r>
</w:p>
Parsed in 0.008 seconds, using
GeSHi 1.0.8.4
docx4j will use the mc:Fallback, so we are interested in:
Using xml Syntax Highlighting
<w:pict>
<v:shapetype id="_x0000_t202" coordsize="21600,21600" o:spt="202" path="m,l,21600r21600,l21600,xe">
<v:stroke joinstyle="miter"/>
<v:path gradientshapeok="t" o:connecttype="rect"/>
</v:shapetype>
<v:shape id="Text Box 2" o:spid="_x0000_s1026" type="#_x0000_t202" style="position:absolute;margin-left:0;margin-top:0;width:186.95pt;height:110.55pt;z-index:251659264;visibility:visible;mso-wrap-style:square;mso-width-percent:400;mso-height-percent:200;mso-wrap-distance-left:9pt;mso-wrap-distance-top:0;mso-wrap-distance-right:9pt;mso-wrap-distance-bottom:0;mso-position-horizontal:center;mso-position-horizontal-relative:text;mso-position-vertical:absolute;mso-position-vertical-relative:text;mso-width-percent:400;mso-height-percent:200;mso-width-relative:margin;mso-height-relative:margin;v-text-anchor:top" o:gfxdata="....">
<v:textbox style="mso-fit-shape-to-text:t">
<w:txbxContent>
<w:p w:rsidR="00F54378" w:rsidRDefault="00F54378">
<w:r>
<w:t>Here are the contents of the text box.
</w:t>
</w:r>
</w:p>
</w:txbxContent>
</v:textbox>
</v:shape>
</w:pict>
Parsed in 0.002 seconds, using
GeSHi 1.0.8.4
2. now change the text box background colour, and font colour. You get:
Using xml Syntax Highlighting
<w:pict>
<v:shapetype id="_x0000_t202" coordsize="21600,21600" o:spt="202" path="m,l,21600r21600,l21600,xe">
<v:stroke joinstyle="miter"/>
<v:path gradientshapeok="t" o:connecttype="rect"/>
</v:shapetype>
<v:shape id="Text Box 2" o:spid="_x0000_s1026" type="#_x0000_t202" style="position:absolute;margin-left:0;margin-top:0;width:186.95pt;height:110.55pt;z-index:251659264;visibility:visible;mso-wrap-style:square;mso-width-percent:400;mso-height-percent:200;mso-wrap-distance-left:9pt;mso-wrap-distance-top:0;mso-wrap-distance-right:9pt;mso-wrap-distance-bottom:0;mso-position-horizontal:center;mso-position-horizontal-relative:text;mso-position-vertical:absolute;mso-position-vertical-relative:text;mso-width-percent:400;mso-height-percent:200;mso-width-relative:margin;mso-height-relative:margin;v-text-anchor:top" fillcolor="yellow">
<v:textbox style="mso-fit-shape-to-text:t">
<w:txbxContent>
<w:p w:rsidR="00F54378" w:rsidRPr="00E82642" w:rsidRDefault="00F54378">
<w:pPr>
<w:rPr>
<w:color w:val="FF0000"/>
</w:rPr>
</w:pPr>
<w:r w:rsidRPr="00E82642">
<w:rPr>
<w:color w:val="FF0000"/>
</w:rPr>
<w:t>Here are the contents of the text box.
</w:t>
</w:r>
</w:p>
</w:txbxContent>
</v:textbox>
</v:shape>
</w:pict>
Parsed in 0.002 seconds, using
GeSHi 1.0.8.4
You can ignore o:gfxdata
You can see the font color is given by <w:color w:val="FF0000"/> and background colour by @fillcolor
So, use one of the approaches described in Getting Started to edit the relevant nodes identified above.
Good luck, and if you need more help, please post your code.