Excel conditional formula

Another point it's hard to test with regex all valid addresses, from A1 to XFD1048576. Expression will be too complex. However, OfficeScript checks itself if we use correct range, thus we may rely on it and use try/catch. In addition it's only to add our own rule which doesn't allow to run script with single cell ranges.  Something like

function main(
    workbook: ExcelScript.Workbook,
    Range_of_references: string = "A2:F2",
    Range_to_color: string = "A3:F3",
    Increment: number = 5
)

// see https://techcommunity.microsoft.com/discussions/excelgeneral/excel-conditional-formula/4464321

 {

    const worksheet = workbook.getActiveWorksheet()
    if (!Range_of_references.includes(":") || !Range_to_color.includes(":")) {
        throw new Error(`Keep ":" in range addresses`)
    }
    try {
        worksheet.getRange(Range_of_references)
        worksheet.getRange(Range_to_color)
    } catch (error) {
        console.log(`Incorrect (${Range_of_references}) or (${Range_to_color}) address. Please inspect and run again.`)
        return
    }

    const sourceRange = worksheet.getRange(Range_of_references)
    const targetRange = worksheet.getRange(Range_to_color)
    const flattenedSource = sourceRange.getValues()[0]
    const incremented = targetRange.getValues()[0].map(x => +x + Increment)

    targetRange.getFormat().getFill().clear()
    Array.from(incremented.keys())
        .filter(n => incremented
            .map(x => flattenedSource.includes(x))[n])
        .map(m => targetRange.getCell(0, m).getFormat().getFill().setColor("lightgreen"))

}

Great approach. Here is with slightly modified second part

function main(
    workbook: ExcelScript.Workbook,
    Range_of_references: string = "A2:F2",
    Range_to_color: string = "A3:F3",
    Increment: number = 5
)

// see https://techcommunity.microsoft.com/discussions/excelgeneral/excel-conditional-formula/4464321

 {

    function normalizeAddress(address: string): string {
        // Trim and convert to uppercase for consistent address validation
        return address.trim().toUpperCase();
    }

    function isValidAddress(address: string): boolean {
        // Basic regex to validate Excel-style cell references (e.g., A1:B3, B10:C10...)
        const regex = /^[A-Z]+[1-9][0-9]*:[A-Z]+[1-9][0-9]*$/;
        return regex.test(normalizeAddress(address));
    }

    // Validate the input address format (e.g., "A1", "B100", "AZ35")
    if (!isValidAddress(Range_of_references) || !isValidAddress(Range_to_color)) {
        // Execution stops on throw (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/throw)
        throw new Error("Please enter valid range address(es) like A1:D1, C4:G4, ...");
    }

    // Get active worksheet and ranges
    const worksheet = workbook.getActiveWorksheet();
    const sourceRange = worksheet.getRange(Range_of_references);
    const targetRange = worksheet.getRange(Range_to_color);


    /////////////////////////////////////////////////////////////////////
    // Another variant
    const flattenedSource = sourceRange.getValues()[0]
    const incremented = targetRange.getValues()[0].map(x => +x + Increment)

    targetRange.getFormat().getFill().clear()
    Array.from( incremented.keys())
            .filter(n => incremented
                .map(x => flattenedSource.includes(x))[n])
            .map( m => targetRange.getCell(0, m).getFormat().getFill().setColor("lightgreen"))
    // end of another variant
    /////////////////////////////////////////////////////////////////////////


/*
    // Count columns in targetRange
    const targetColumns: number = targetRange.getColumnCount();

    // Flatten sourceRange (workaround to .flat() that raises a warning in Excel TS console)
    const flattenedSource = sourceRange.getValues()
        .reduce((acc, row) => acc.concat(row), []);

    // Iterate cells in targetRange
    for (let i = 0; i <= targetColumns; i++) {
        const currentCell = targetRange.getCell(0, i);
        // Invoking read methods inside of a loop could lead to slow perf.:
        const currentValue = Number(currentCell.getValue());

        // Iterate flattenedSource
        for (let j = 0; j <= targetColumns; j++) {
            // If currentValue (in targetRange) + Increment = current flattenedSource value...
            if (flattenedSource[j] === currentValue + Increment) {
                // Color the cell in targetRange
                currentCell.getFormat().getFill().setColor("lightgreen")  //   ("#C1F0C8");
                break
            }
        }
    }
*/

}