diff --git a/histogram_internal.h b/histogram_internal.h
index 7473744..2cc39b9 100644
--- a/histogram_internal.h
+++ b/histogram_internal.h
@@ -1,4 +1,5 @@
 #ifndef HISTOGRAM_INTERNAL_H_INCLUDED
 #define HISTOGRAM_INTERNAL_H_INCLUDED
 void find_minmax(const std::vector<double>& numbers, double& min, double& max);
+std::string check_color(const std::string& color);
 #endif // HISTOGRAM_INTERNAL_H_INCLUDED
diff --git a/svg.cpp b/svg.cpp
index 84ddc82..1ec5992 100644
--- a/svg.cpp
+++ b/svg.cpp
@@ -2,7 +2,6 @@
 #include <vector>
 #include "svg.h"
 
-
 void svg_begin(double width, double height)
 {
     std::cout << "<?xml version='1.0' encoding='UTF-8'?>\n";
diff --git a/unittest.cpp b/unittest.cpp
index 2b818d9..9c59d84 100644
--- a/unittest.cpp
+++ b/unittest.cpp
@@ -3,6 +3,7 @@
 #include "doctest.h"
 #include "histogram_internal.h"
 
+
 TEST_CASE("distinct positive numbers") {
     double min = 0;
     double max = 0;
@@ -42,3 +43,15 @@ TEST_CASE("equal elements") {
     CHECK(min == 4);
     CHECK(max == 4);
 }
+
+TEST_CASE("correct input 1") {
+    CHECK(check_color("red") == "red");
+}
+
+TEST_CASE("uncorrect input 2") {
+    CHECK(check_color("light red") == "black");
+}
+
+TEST_CASE("uncorrect input 2") {
+    CHECK(check_color("FF0000") == "black");
+}