Use Instagram's built in colour picker tool on Stories to keep your brand looking consistent.
Upload a background which contains one of your brand colours.
Type out some text.
Press on the colourful button at the top to change the text colour.
Click on the eye dropper tool to the left of the colour palette.
Move the cursor to select your brand colour.
Toggle with the background text colour button to change the colour for the text or the background.
See the simple video below to show the above steps in action.
Let me know in the comments if you try it!